A Manual of Elementary Geology Part 50

_Note._ The dotted lines at _i_ and _k_ express portions of rock removed by denudation, the amount of which may be estimated by supposing similar lines prolonged from other points where different strata end abruptly at the surface.

_N.B._ The lower section at ** joins on to the upper one at *.]

Starting from the sh.o.r.es of the Atlantic, on the eastern side of the Continent, we first come to a low region (A B), which was called the alluvial plain by the first geographers. It is occupied by tertiary and cretaceous strata, before described (pp. 171. 206. and 224.), which are nearly horizontal. The next belt, from B to C, consists of granitic rocks (hypogene), chiefly gneiss and mica-schist, covered occasionally with unconformable red sandstone, No. 4. (New Red or Trias?), remarkable for its ornithichnites (see p. 327.). Sometimes, also, this sandstone rests on the edges of the disturbed paleozoic rocks (as seen in the section). The region (B C), sometimes called the "Atlantic Slope," corresponds nearly in average width with the low and flat plain (A, B), and is characterized by hills of moderate height, contrasting strongly, in their rounded shape and alt.i.tude, with the long, steep, and lofty parallel ridges of the Alleghany mountains.

The outcrop of the strata in these ridges, like the two belts of hypogene and newer rocks (A B, and B C), above alluded to, when laid down on a geological map, exhibit long stripes of different colours, running in a N.E. and S.W. direction, in the same way as the lias, chalk, and other secondary formations in the middle and eastern half of England.

The narrow and parallel zones of the Appalachians here mentioned, consist of strata, folded into a succession of convex and concave flexures, subsequently laid open by denudation. The component rocks are of great thickness, all referable to the Silurian, Devonian, and Carboniferous formations. There is no princ.i.p.al or central axis, as in the Pyrenees and many other chains--no nucleus to which all the minor ridges conform; but the chain consists of many nearly equal and parallel foldings, having what is termed an anticlinal and synclinal arrangement (see above, p. 48.). This system of hills extends, geologically considered, from Vermont to Alabama, being more than 1000 miles long, from 50 to 150 miles broad, and varying in height from 2000 to 6000 feet. Sometimes the whole a.s.semblage of ridges runs perfectly straight for a distance of more than 50 miles, after which all of them wheel round together, and take a new direction, at an angle of 20 or 30 degrees to the first.

We are indebted to the state surveyors of Virginia and Pennsylvania, Prof.

W. B. Rogers and his brother Prof. H. D. Rogers, for the important discovery of a clue to the general law of structure prevailing throughout this range of mountains, which, however simple it may appear when once made out and clearly explained, might long have been overlooked; amidst so great a ma.s.s of complicated details. It appears that the bending and fracture of the beds is greatest on the south-eastern or Atlantic side of the chain, and the strata become less and less disturbed as we go westward, until at length they regain their original or horizontal position. By reference to the section (fig. 379.), it will be seen that on the eastern side, or in the ridges and troughs nearest the Atlantic, south-eastern dips predominate, in consequence of the beds having been folded back upon themselves, as in _i_, those on the north-western side of each arch having been inverted. The next set of arches (such as _k_) are more open, each having its western side steepest; the next (_l_) opens out still more widely, the next (_m_) still more, and this continues until we arrive at the low and level part of the Appalachian coal-field (D E).

In nature or in a true section, the number of bendings or parallel folds is so much greater that they could not be expressed in a diagram without confusion. It is also clear that large quant.i.ties of rock have been removed by aqueous action or denudation, as will appear if we attempt to complete all the curves in the manner indicated by the dotted lines at _i_ and _k_.

The movements which imparted so uniform an order of arrangement to this vast system of rocks must have been, if not contemporaneous, at least parts of one and the same series, depending on some common cause. Their geological date is well defined, at least within certain limits, for they must have taken place after the deposition of the carboniferous strata (No. 5.), and before the formation of the red sandstone (No. 4.).

The greatest disturbing and denuding forces have evidently been exerted on the south-eastern side of the chain; and it is here that igneous or plutonic rocks are observed to have invaded the strata, forming d.y.k.es, some of which run for miles in lines parallel to the main direction of the Appalachians, or N.N.E. and S.S.W.

The thickness of the carboniferous rocks in the region C is very great, and diminishes rapidly as we proceed to the westward. The surveys of Pennsylvania and Virginia show that the south-east was the quarter whence the coa.r.s.er materials of these strata were derived, so that the ancient land lay in that direction. The conglomerate which forms the general base of the coal-measures is 1500 feet thick in the Sharp Mountain, where I saw it (at C) near Pottsville; whereas it has only a thickness of 500 feet, about thirty miles to the north-west, and dwindles gradually away when followed still farther in the same direction, till its thickness is reduced to 30 feet.[329-A] The limestones, on the other hand, of the coal-measures, augment as we trace them westward. Similar observations have been made in regard to the Silurian and Devonian formations in New York; the sandstones and all the mechanically-formed rocks thinning out as they go westward, and the limestones thickening, as it were, at their expense. It is, therefore, clear that the ancient land was to the east, where the Atlantic now is; the deep sea, with its banks of coral and sh.e.l.ls to the west, or where the hydrographical basin of the Mississippi is now situated.

In that region, near Pottsville, where the thickness of the coal-measures is greatest, there are thirteen seams of anthracitic coal, several of them more than 2 yards thick. Some of the lowest of these alternate with beds of white grit and conglomerate of coa.r.s.er grain than I ever saw elsewhere, a.s.sociated with pure coal. The pebbles of quartz are often of the size of a hen's egg. On following these pudding-stones and grits for several miles from Pottsville, by Tamaqua, to the Lehigh Summit Mine, in company with Mr.

H. D. Rogers, in 1841, he pointed out to me that the coa.r.s.e-grained strata and their accompanying shales gradually thin out, until seven seams of coal, at first widely separated, are brought nearer and nearer together, until they successively unite; so that at last they form one ma.s.s, between 40 and 50 feet thick. I saw this enormous bed of anthracitic coal quarried in the open air at Mauch Chunk (or the Bear Mountain), the overlying sandstone, 40 feet thick, having been removed bodily from the top of the hill, which, to use the miner's expression, had been "scalped." The acc.u.mulation of vegetable matter now const.i.tuting this vast bed of anthracite, may perhaps, before it was condensed by pressure and the discharge of its hydrogen, oxygen, and other volatile ingredients, have been between 200 and 300 feet thick. The origin of such a vast thickness of vegetable remains, so unmixed with earthy ingredients, can, I think, be accounted for in no other way, than by the growth, during thousands of years, of trees and ferns, in the manner of peat,--a theory which the presence of the Stigmaria _in situ_ under each of the seven layers of anthracite, fully bears out. The rival hypothesis, of the drifting of plants into a sea or estuary, leaves the absence of sediment, or, in this case, of sand and pebbles, wholly unexplained.

[Ill.u.s.tration: Fig. 380. Cross section.]

[Ill.u.s.tration: Fig. 381. Cross section.]

But the student will naturally ask, what can have caused so many seams of coal, after they had been persistent for miles, to come together and blend into one single seam, and that one equal in the aggregate to the thickness of the several separate seams? Often had the same question been put by English miners before a satisfactory answer was given to it by the late Mr.

Bowman. The following is his solution of the problem. Let _a a'_, fig.

380., be a ma.s.s of vegetable matter, capable, when condensed, of forming a 3-foot seam of coal. It rests on the underclay _b b'_, filled with roots of trees _in situ_, and it supports a growing forest (C D). Suppose that part of the same forest D E had become submerged by the ground sinking down 25 feet, so that the trees have been partly thrown down and partly remain erect in water, slowly decaying, their stumps and the lower parts of their trunks being enveloped in layers of sand and mud, which are gradually filling up the lake D F. When this lake or lagoon has at length been entirely silted up and converted into land, say, in the course of a century, the forest C D will extend once more continuously over the whole area C F, as in fig. 381., and another ma.s.s of vegetable matter (_g g'_), forming 3 feet more of coal, may acc.u.mulate from C to F. We then find in the region F, two seams of coal (_a'_ and _g'_) each 3 feet thick, and separated by 25 feet of sandstone and shale, with erect trees based upon the lower coal, while, between D and C, we find these two seams united into a 2-yard coal. It may be objected that the uninterrupted growth of plants during the interval of a century will have caused the vegetable matter in the region C D to be thicker than the two distinct seams _a'_ and _g'_ at F; and no doubt there would actually be a slight excess representing one generation of trees with the remains of other plants, forming half an inch or an inch of coal; but this would not prevent the miner from affirming that the seam _a g_, throughout the area C D, was equal to the two seams _a'_ and _g'_ at F.

The reader has seen, by reference to the section (fig. 379. p. 327.), that the strata of the Appalachian coal-field a.s.sume an horizontal position west of the mountains. In that less elevated country, the coal-measures are intersected by three great navigable rivers, and are capable of supplying for ages, to the inhabitants of a densely peopled region, an inexhaustible supply of fuel. These rivers are the Monongahela, the Alleghany, and the Ohio, all of which lay open on their banks the level seams of coal. Looking down the first of these at Brownsville, we have a fine view of the main seam of bituminous coal 10 feet thick, commonly called the Pittsburg seam, breaking out in the steep cliff at the water's edge; and I made the accompanying sketch of its appearance from the bridge over the river (see fig. 382.). Here the coal, 10 feet thick, is covered by carbonaceous shale (_b_), and this again by micaceous sandstone (_c_). Horizontal galleries may be driven everywhere at very slight expense, and so worked as to drain themselves, while the cars, laden with coal and attached to each other, glide down on a railway, so as to deliver their burden into barges moored to the river's bank. The same seam is seen at a distance, on the right bank (at _a_), and may be followed the whole way to Pittsburg, fifty miles distant. As it is nearly horizontal, while the river descends it crops out at a continually increasing, but never at an inconvenient, height above the Monongahela. Below the great bed of coal at Brownsville is a fire-clay 18 inches thick, and below this, several beds of limestone, below which again are other coal seams. I have also shown in my sketch another layer of workable coal (at _d d_), which breaks out on the slope of the hills at a greater height. Here almost every proprietor can open a coal-pit on his own land, and the stratification being very regular, he may calculate with precision the depth at which coal may be won.

The Appalachian coal-field, of which these strata form a part (from C to E, section, fig. 379., p. 327.), is remarkable for its vast area; for, according to Professor H. D. Rogers, it stretches continuously from N.E. to S.W., for a distance of 720 miles, its greatest width being about 180 miles. On a moderate estimate, its superficial area amounts to 63,000 square miles.

[Ill.u.s.tration: Fig. 382. View of the great Coal Seam on the Monongahela at Brownsville, Pennsylvania, U. S.

_a._ Ten-foot seam of coal.

_b._ Black bituminous or carbonaceous shale, 10 feet thick.

_c._ Micaceous sandstone.

_d d._ Upper seam of coal, 6 feet thick.]

This coal formation, before its original limits were reduced by denudation, must have measured 900 miles in length, and in some places more than 200 miles in breadth. By again referring to the section (fig. 379., p.

327.), it will be seen that the strata of coal are horizontal to the westward of the mountains in the region D E, and become more and more inclined and folded as we proceed eastward. Now it is invariably found, as Professor H. D. Rogers has shown by chemical a.n.a.lysis, that the coal is most bituminous towards its western limit, where it remains level and unbroken, and that it becomes progressively debituminized as we travel south-eastward towards the more bent and distorted rocks. Thus, on the Ohio, the proportion of hydrogen, oxygen, and other volatile matters, ranges from forty to fifty per cent. Eastward of this line, on the Monongahela, it still approaches forty per cent., where the strata begin to experience some gentle flexures. On entering the Alleghany Mountains, where the distinct anticlinal axes begin to show themselves, but before the dislocations are considerable, the volatile matter is generally in the proportion of eighteen or twenty per cent. At length, when we arrive at some insulated coal-fields (5', fig. 379.) a.s.sociated with the boldest flexures of the Appalachian chain, where the strata have been actually turned over, as near Pottsville, we find the coal to contain only from six to twelve per cent. of bitumen, thus becoming a genuine anthracite.[333-A]

It appears from the researches of Liebig and other eminent chemists, that when wood and vegetable matter are buried in the earth, exposed to moisture, and partially or entirely excluded from the air, they decompose slowly and evolve carbonic acid gas, thus parting with a portion of their original oxygen. By this means, they become gradually converted into lignite or wood-coal, which contains a larger proportion of hydrogen than wood does. A continuance of decomposition changes this lignite into common or bituminous coal, chiefly by the discharge of carburetted hydrogen, or the gas by which we illuminate our streets and houses. According to Bischoff, the inflammable gases which are always escaping from mineral coal, and are so often the cause of fatal accidents in mines, always contain carbonic acid, carburetted hydrogen, nitrogen, and olefiant gas.

The disengagement of all these gradually transforms ordinary or bituminous coal into anthracite, to which the various names of splint coal, glance coal, culm, and many others, have been given.

We have seen that, in the Appalachian coal-field, there is an intimate connection between the extent to which the coal has parted with its gaseous contents, and the amount of disturbance which the strata have undergone. The coincidence of these phenomena may be attributed partly to the greater facility afforded for the escape of volatile matter, where the fracturing of the rocks had produced an infinite number of cracks and crevices, and also to the heat of the gases and water penetrating these cracks, when the great movements took place, which have rent and folded the Appalachian strata. It is well known that, at the present period, thermal waters and hot vapours burst out from the earth during earthquakes, and these would not fail to promote the disengagement of volatile matter from the carboniferous rocks.

_Continuity of seams of coal._--As single seams of coal are continuous over very wide areas, it has been asked, how forests could have prevailed uninterruptedly over such wide s.p.a.ces, without being oftener flooded by turbid rivers, or, when submerged, denuded by marine currents. It appears, from the description of the Cape Breton coal-field, by Mr. Richard Brown, that false stratification is common in the beds of sand, and some partial denudation of these, at least, must often have taken place during the acc.u.mulation of the carboniferous series.

In the Forest of Dean, ancient river-channels are found, which pa.s.s through beds of coal, and in which rounded pebbles of coal occur. They are of older date than the overlying and undisturbed coal-measures. The late Mr. Buddle, who described them to me, told me he had seen similar phenomena in the Newcastle coal-field. Nevertheless, instances of these channels are much more rare than we might have antic.i.p.ated, especially when we remember how often the roots of trees (_Stigmariae_) have been torn up, and drifted in broken fragments into the grits and sandstones. The prevalence of a downward movement is, no doubt, the princ.i.p.al cause which has saved so many extensive seams of coal from destruction by fluviatile action.

The purity of the coal, or its non-intermixture with earthy matter, presents another theoretical difficulty to many geologists, who are inclined to believe that the trees and smaller plants of the carboniferous period grew in extensive swamps, rather than on land not liable to be inundated. It appears, however, that in the alluvial plain and delta of the Mississippi, extensive "cypress swamps," as they are called, densely covered with various trees, occur, into which no matter held in mechanical suspension is ever introduced during the greatest inundations, inasmuch as they are all surrounded by a dense marginal belt of reeds, canes, and brushwood. Through this thick barrier the river-water must pa.s.s, so that it is invariably well filtered before it can reach the interior of the forest-covered area, within which, vegetable matter is continually acc.u.mulating from the decay of trees and semi-aquatic plants. In proof of this, I may observe, that whenever any part of a swamp is dried up, during an unusually hot season, and the wood set on fire, pits are burnt into the ground many feet deep, or as far down as the fire can descend without meeting with water, and it is then found that scarcely any residuum or earthy matter is left.[334-A]

At the bottom of these "cypress swamps" of the Mississippi, a bed of clay is found, with roots of the tall cypress (_Taxodium distichum_), just as the underclays of the coal are filled with _Stigmaria_.

_Climate of Coal Period._--So long as the botanist taught that a tropical climate was implied by the carboniferous flora, geologists might well be at a loss to reconcile the preservation of so much vegetable matter with a high temperature; for heat hastens the decomposition of fallen leaves and trunks of trees, whether in the atmosphere or in water.[335-A] It is well known that peat, so abundant in the bogs of high lat.i.tudes, ceases to grow in the swamps of warmer regions. It seems, however, to have become a more and more received opinion, that the coal-plants do not, on the whole, indicate a climate resembling that now enjoyed in the equatorial zone.

Tree-ferns range as far south as the southern part of New Zealand, and Araucarian pines occur in Norfolk Island. A great predominance of ferns and lycopodiums indicates warmth, moisture, equability of temperature, and freedom from frost, rather than intense heat; and we know too little of the sigillariae, calamites, asterophyllites, and other peculiar forms of the carboniferous period, to be able to speculate with confidence on the kind of climate they may have required.

No doubt, we are ent.i.tled to presume, from the corals and cephalopoda of the mountain limestone, that a warm temperature characterized the northern seas in the carboniferous era; but the absence of cold may have given rise (as at present in the seas of the Bermudas, under the influence of the gulf stream) to a very wide geographical range of stone-building corals and sh.e.l.l-bearing cuttle-fish, without its being necessary to call in the aid of tropical heat.[335-B]

CARBONIFEROUS REPTILES.

Where we have evidence in a single coal-field, as in that of Nova Scotia, or South Wales, of fifty or even a hundred ancient forests buried one above the other, with the roots of trees still in their original position, and with some of the trunks still remaining erect, we are apt to wonder that until the year 1844 no remains of contemporaneous air-breathing creatures, except a few insects, had been discovered. No vertebrated animals more highly organized than fish, no mammalia or birds, no saurians, frogs, tortoises, or snakes, were yet known in rocks of such high antiquity. In the coal-field of Coalbrook Dale mention had been made of two species of beetles of the family _Curculionidae_, and of a neuropterous insect resembling the genus _Corydalis_, with another related to the _Phasmidae_.[335-C] In other coal-measures in Europe we find notice of a scorpion and of a moth allied to _Tinea_, also of one air-breathing crustacean, or land-crab. Yet Aga.s.siz had already described in his great work on fossil fishes more than one hundred and fifty species of ichthyolites from the coal strata, ninety-four belonging to the families of shark and ray, and fifty-eight to the cla.s.s of ganoids. Some of these fish are very remote in their organization from any now living, especially those of the family called _Sauroid_ by Aga.s.siz; as _Megalichthys_, _Holoptychius_, and others, which are often of great size, and all predaceous. Their osteology, says M. Aga.s.siz, reminds us in many respects of the skeletons of saurian reptiles, both by the close sutures of the bones of the skull, their large conical teeth striated longitudinally (see fig. 383.), the articulations of the spinous processes with the vertebrae, and other characters. Yet they do not form a family intermediate between fish and reptiles, but are true _fish_, though doubtless more highly organized than any living fish.[336-A]

[Ill.u.s.tration: Fig. 383. _Holoptychius Hibberti_, Ag. Fifes.h.i.+re coal-field; natural size.]

The annexed figure represents a large tooth of the _Megalichthys_, found by Mr. Horner in the Cannel coal of Fifes.h.i.+re. It probably inhabited an estuary, like many of its contemporaries, and frequented both rivers and the sea.

[Ill.u.s.tration: Fig. 384. _Archegosaurus minor_, Goldfuss. Fossil reptile from the coal-measures, Saarbruck.]

At length, in 1844, the first skeleton of a true reptile was announced from the coal of Munster-Appel in Rhenish Bavaria, by H. von Meyer, under the name of _Apateon pedestris_, the animal being supposed to be nearly related to the salamanders. Three years later, in 1847, Prof. von Dechen found in the coal-field of Saarbruck, at the village of Lebach, between Strasburg and Treves, the skeletons of no less than three distinct species of air-breathing reptiles, which were described by the late Prof. Goldfuss under the generic name of _Archegosaurus_. The ichthyolites and plants found in the same strata, left no doubt that these remains belonged to the true coal period. The skulls, teeth, and the greater portions of the skeleton, nay, even a large part of the skin, of two of these reptiles have been faithfully preserved in the centre of spheroidal concretions of clay-iron-stone. The largest of these lizards, _Archegosaurus Decheni_, must have been 3 feet 6 inches long. The annexed drawing represents the smallest of the three of the natural size. They were considered by Goldfuss as saurians, but by Herman von Meyer as most nearly allied to the _Labyrinthodon_, and therefore connected with the batrachians, as well as the lizards. The remains of the extremities leave no doubt that they were quadrupeds, "provided," says Von Meyer, "with hands and feet terminating in distinct toes; but these limbs were weak, serving only for swimming or creeping." The same anatomist has pointed out certain points of a.n.a.logy between their bones and those of the _Proteus anguinus_; and Mr. Owen has observed to me that they make an approach to the _Proteus_ in the shortness of their ribs. Two of these ancient reptiles retain a large part of the outer skin, which consisted of long, narrow, wedge-shaped, tile-like, and h.o.r.n.y scales, arranged in rows (see fig. 385.).

[Ill.u.s.tration: Fig. 385. Imbricated covering of skin of _Archegosaurus medius_, Goldf.; magnified.[337-A]]

_Cheirotherian footprints in coal measures, United States._--In 1844, the very year when the Apateon or Salamander of the coal was first met with in the country between the Moselle and the Rhine, Dr. King published an account of the footprints of a large reptile discovered by him in North America. These occur in the coal strata of Greensburg, in Westmoreland County, Pennsylvania; and I had an opportunity of examining them in 1846. I was at once convinced of their genuineness, and declared my conviction on that point, on which doubts had been entertained both in Europe and the United States. The footmarks were first observed standing out in relief from the lower surface of slabs of sandstone, resting on thin layers of fine unctuous clay. I brought away one of these ma.s.ses, which is represented in the accompanying drawing (fig.

386.). It displays, together with footprints, the casts of cracks (_a_, _a'_) of various sizes. The origin of such cracks in clay, and casts of the same, has before been explained, and referred to the drying and shrinking of mud, and the subsequent pouring of sand into open crevices.

It will be seen that some of the cracks, as at _b_, _c_, traverse the footprints, and produce distortion in them, as might have been expected, for the mud must have been soft when the animal walked over it and left the impressions; whereas, when it afterwards dried up and shrank, it would be too hard to receive such indentations.

No less than twenty-three footsteps were observed by Dr. King in the same quarry before it was abandoned, the greater part of them so arranged (see fig. 387.) on the surface of one stratum as to imply that they were made successively by the same animal. Everywhere there was a double row of tracks, and in each row they occur in pairs, each pair consisting of a hind and fore foot, and each being at nearly equal distances from the next pair. In each parallel row the toes turn the one set to the right, the other to the left. In the European _Cheirotherium_, before mentioned (p. 290.), both the hind and fore feet have each five toes, and the size of the hind foot is about five times as large as the fore foot. In the American fossil the posterior footprint is not even twice as large as the anterior, and the number of toes is unequal, being five in the hinder and four in the anterior foot.

In this, as in the European _Cheirotherium_, one toe stands out like a thumb, and these thumb-like toes turn the one set to the right, and the other to the left. The American _Cheirotherium_ was evidently a broader animal, and belonged to a distinct genus from that of the tria.s.sic age in Europe.[338-A]

[Ill.u.s.tration: Fig. 386. _Scale one-sixth the original._ Slab of sandstone from the coal-measures of Pennsylvania, with footprints of air-breathing reptile and casts of cracks.]

We may a.s.sume that the reptile which left these prints on the ancient sands of the coal-measures was an air-breather, because its weight would not have been sufficient under water to have made impressions so deep and distinct. The same conclusion is also borne out by the casts of the cracks above described, for they show that the clay had been exposed to the air and sun, so as to have dried and shrunk.

[Ill.u.s.tration: Fig. 387. Series of reptilian footprints in the coal-strata of Westmoreland County, Pennsylvania.

_a._ Mark of nail?]

The geological position of the sandstone of Greensburg is perfectly clear, being situated in the midst of the Appalachian coal-field, having the main bed of coal, called the Pittsburg seam, above mentioned (p. 331.), 3 yards thick, 100 feet above it, and worked in the neighbourhood, with several other seams of coal at lower levels. The impressions of _Lepidodendron_, _Sigillaria_, _Stigmaria_, and other characteristic carboniferous plants, are found both above and below the level of the reptilian footsteps.

a.n.a.logous footprints of a large reptile of still older date have since been found (1849), by Mr. Isaac Lea, in the lowest beds of the coal formation at Pottsville, near Philadelphia, so that we may now be said to have the footmarks of two reptilians of the coal period, and the skeletons of four.[340-A]

CARBONIFEROUS OR MOUNTAIN LIMESTONE.

We have already seen that this rock lies sometimes entirely beneath the coal-measures, while, in other districts, it alternates with the shales and sandstone of the coal. In both cases it is dest.i.tute of land plants, and usually charged with corals, which are often of large size; and several species belong to the lamelliferous cla.s.s of Lamarck, which enter largely into the structure of coral reefs now growing. There are also a great number of _Crinoidea_ (see fig. 388.), and a few _Echinoderms_, a.s.sociated with the zoophytes above mentioned. The _Brachiopoda_ const.i.tute a large proportion of the Mollusca, many species being referable to two extinct genera, _Spirifer_ (or _Spirifera_) (fig. 389.), and _Productus_ (_Leptaena_) (fig. 390.).