The Solomon Islands and Their Natives Part 43

CHAPTER XVI.

LAND AND FRESH-WATER Sh.e.l.lS.

DURING my numerous excursions in these islands, I had in the majority of instances to follow up the stream-courses in order to examine their geological structure. I had therefore good opportunities in these regions of making a collection of the fresh-water sh.e.l.ls, which, together with the land sh.e.l.ls I collected, formed a total of between sixty and seventy species, amongst which there were 11 new species and at least 5 new varieties, whilst about 14 would appear to have been never previously recorded from the Solomon Islands, and there were in addition several from new localities in the group. The collection was sent to the British Museum and was examined and described by Mr. E.

Smith, to whose paper on the subject[479] I am indebted for my acquaintance with the sh.e.l.ls in question, and through whose kindness I have been thus enabled to supplement my other observations in these islands. A list of the sh.e.l.ls with the descriptions of the new species is given on page 344. For its size, my collection presented a large amount of novelty, coming as it did from a region the land and fresh-water sh.e.l.ls of which were previously considered to be fairly known. There can be no doubt, however, that in the Solomon Islands the conchologist has much work that remains to be done. Not only are the higher regions of the larger islands, entirely unexplored, but it would appear from the collections made up to the present date in this large group, that particular species may be not only confined to a special sub-group of islands but may be restricted to a single island, and that other species more widely distributed through the group may be represented in each island and in different districts of the larger islands by different varieties. Had I been aware of the extent of the influence of locality in this region, I might have made my collection of greater value. It would therefore seem necessary for future collectors in this group to make in every small island and in different districts of the larger islands special independent collections, disregarding the fact that they may have apparently met with the same sh.e.l.l very frequently before, because many of the varieties and some of the species can only be distinguished by the practised eye of the specialist, and a new locality for a previously well-known species may be often unwittingly found.

[479] "On a collection of sh.e.l.ls from the Solomon Islands" by Edgar A. Smith. (Proceedings of the Zoological Society: June 2nd, 1885.) This paper is ill.u.s.trated with two coloured plates of the new sh.e.l.ls.

As an instance of the unexpected results, which may fall to the lot of others in this group, I may here add, that out of eleven land and fresh-water sh.e.l.ls that I collected in the small island of Santa Anna, which is only 2 miles in length, four were new species, and besides there were some new varieties. The stations of these four species may be suggestive. Two of them--_Helix_ (_Videna_) _sanctae annae_ and _Helix_ (_nanina_) _solidiuscula_--were generally found on the trunks of the cocoa-nut palms at the coast; whilst the other two occurred in situations far more likely to yield new species, _Melania sanctae annae_ being obtained from a small stream in the interior of the island, and _Melania guppyi_ being found dead in the stomach and intestines of a fish that frequents the fresh-water lake of Wailava. This last sh.e.l.l would appear to live in the deeper parts of the lake, as I only found one living specimen, all the others being obtained from the stomach and intestines of these fish. Mr. Smith describes it as "a very remarkable and distinct species." Its length is about 1? inches; and its sharp-pointed spire was to be sometimes seen protruding through the vent of the fish, which evidently digests the animal and ejects the sh.e.l.l.

These fish were usually 9 or 10 inches long; but the full-grown sh.e.l.ls were found also in fish half this size, when the relation between the length of the sh.e.l.l and the size of the fish was truly alarming. Since the little fish actually swallow sharp pointed sh.e.l.ls measuring a fifth of their own length and pa.s.s them out through the vent after they have digested the animal, we must credit them with a remarkable capacity for adapting their diet to circ.u.mstances.

To exemplify the variation which some species of sh.e.l.ls display in this group, I will take the instance of _Helix_ (_Geotrochus_) _cleryi_, Recluz. This species is probably distributed through the whole group; but considerable variation prevails in different islands. Amongst the several forms which I obtained, three were named as new varieties, var.

_meridionalis_ from Santa Anna, var. _simboana_ from Simbo or Eddystone Island, and var. _septentrionalis_ from the islands of Bougainville Straits, the localities of the two last varieties being only 80 miles apart. Mr. Smith remarks that this species is "subject to considerable variation in size, colour, and form, apparently resulting from difference of habitat... ... ... Whether these several varieties should take specific rank is questionable, for, although there is a considerable difference between the extreme forms, even in the series of nearly one hundred specimens under examination, the gradual transition from one form to another is observable."

Amongst the more singular in appearance of the land sh.e.l.ls, I may refer to the large _Bulimus_ (_B. cleryi_) which I found on the north coast of St. Christoval. It attains a length of four inches. I was never able to get a living specimen, as they are said by the natives to live in the foliage of the high trees. The specimens which I obtained were empty sh.e.l.ls which the natives of the Koofeh district on the north coast of St. Christoval are in the habit of throwing into heaps, each man when he picks up a sh.e.l.l throwing it into the next heap he pa.s.ses. I was unable to learn the reason of this practice and the natives did not seem willing to tell me... . Two other _Bulimi_ I commonly met with. One was the pretty _Bulimus miltocheilus_ (Reeve), which, when the animal is young and the sh.e.l.l delicate, has a greenish-yellow hue resembling the colour of the leaves it feeds upon: as it grows older the sh.e.l.l becomes thicker and stronger, and in proportion as there is less need for protective resemblance, the greenish-yellow hue fades away, leaving a dull white colour behind. This species is found in St. Christoval and the adjacent islands. The other _Bulimus_ (_B. founaki_, Homb. Jacq.) which I found in Faro Island, Bougainville Straits, and which had been only previously obtained at Isabel Island, attains a length of rather under three inches.

I come now to refer to the fresh-water sh.e.l.ls of these regions. Stated in their order of frequency, the _Neritinae_, _Melaniae_, and _Navicellae_ are the common fresh-water sh.e.l.ls of these islands. The _Neritinae_ were especially interesting to me. They abound in the streams: some of them preferring the moist rocks above the water, others finding their home in the waters of a quiet pool, whilst others, like the _Navicellae_, prefer to buffet the full rush of the torrent. An important feature with reference to these fresh-water Nerites or _Neritinae_ is their wide dispersal. "Some of these species"--as Mr. Smith remarks in respect of those in my collection--"range not only through most of the islands of the Solomon Group, but have a considerably wider distribution." Thus, _Neritina subsulcata_ (Sowerby) and _N. cornea_ (Linne), are not only found in the Solomon Islands, but also occur in the Philippines: _N.

macgillivrayi_ (Reeve) and _N. pet.i.ti_ (Recluz) alike exist in the Fiji and in the Solomon Groups; while _N. porcata_ (Gould) has been found in Samoa and in Fiji as well as in the opposite extremities of the group with which I am at present concerned. Being interested in the question of the mode of dispersal of these Nerites, I made the following experiment to test their powers of sustaining submersion in salt-water.

One individual belonging to the species _Neritina subsulcata_[480]--a species which is also found as above stated in the Philippines, and at the same time is the most widely dispersed fresh-water Nerite in the Solomon Group--survived a submersion of twelve hours; but not one out of a dozen individuals was found alive after a submersion of five days, although the water was changed from time to time. The result was a surprise to me, as I inferred from the result of Baron Aucapitaine's experiments as related by Mr. Darwin,[481] that their close-fitting stony _opercula_ would have enabled them to resist the action of salt-water. Their death could have been scarcely due to want of food, since I have kept sh.e.l.ls of this species for several months on a very scanty diet, and since the powers of endurance of other fresh-water sh.e.l.ls are well known. The matter pa.s.sed out of my mind until after my arrival in England, when Mr. Smith put the question to me, as to their mode of dispersal. I then remembered that their calcareous egg-capsules, which are so commonly seen on the rocky sides of the streams, are in all probability sufficiently thick to resist the action of salt-water. Here is therefore a probable mode of dispersal, and I see it is one which Mr.

Smith refers to as such in his paper. These egg-capsules "if attached to floating timber, might be carried to considerable distances." They are often to be observed on the outside of the sh.e.l.ls of living _Navicellae_, and I have seen them on the backs of the valves of a _Unio_ which I discovered in the Shortland Islands.

[480] In his paper Mr. Smith refers to the species experimented on as _N. cornea_: but in my own list he named a sh.e.l.l belonging to one of the _Neritinae_ in question as _N. subsulcata_.

[481] _Cyclostoma elegans_ was the species tested: _vide_ "Origin of Species," p. 353, 6th edit.

One common feature of these fresh-water sh.e.l.ls, whether _Neritinae_, _Navicellae_, or _Melaniae_, is the extensive erosion of the apices and surrounding parts of the sh.e.l.ls. In some instances I have noticed that almost the entire exterior of the sh.e.l.l has been extensively eroded, particularly in the case of _Neritina subsulcata_, but I always found that the erosion was greatest in non-calcareous districts, where the free carbonic acid in the water is not all consumed in the solution of the limestone rocks. In volcanic islands the erosion of the fresh-water sh.e.l.ls is greater than in islands of calcareous formation; and in streams, which, like those of the north coast of St. Christoval, flow in the upper portion of their course through a district of volcanic rocks and in the lower portion through a district of calcareous rocks, the same difference in the degree of erosion may be observed. I learn from a recent work by Professor Semper[482] that it is the boring of a minute fungus which first exposes the calcareous substance to the action of the carbonic acid, and that the mechanical action of the stream in forming tiny whirl-pools in the cavity probably a.s.sists in the erosion.

[482] "The Natural Conditions of Existence, etc:" London 1881: p.

212, circa.

There are two common species of _Neritina_ in these islands which I often confounded, viz., _N. subsulcata_ and _N. cornea_; and I learn from Mr. Smith's paper that these two species very closely approach each other. They, however, are usually to be found in different stations, _N.

cornea_ occurring on the trunks of palms and other trees away from the streams,[483] and _N. subsulcata_ preferring the moist rocky sides of the streams a foot or so above the water.[484] Now and then they may be found encroaching on each other's domain; for I have found them together on the trunks and branches of areca palms and tree-ferns in low lying moist districts, whilst, as at Choiseul Bay, I found them together in the streams.[485] Now it is a significant circ.u.mstance, that the specimen of _N. cornea_ in my collection which was found by Mr. Smith to make the nearest approach to _N. subsulcata_ was one which I obtained from a stream in Choiseul Bay. It had, in this case, not only intruded on the station of _N. subsulcata_, but had also a.s.sumed some of the distinctive characters of that species. It, therefore, seems to me probable that a graduated series of the sh.e.l.ls of these two species might be formed, which would present the stages of transition from the one species to the other. If this be possible, then I would suggest that the fresh-water Nerite (_Neritina subsulcata_) may have been transformed into the tree Nerite (_Neritina cornea_) in the following manner.

[483] In St. Christoval I found this species on one occasion 150 feet above the nearest stream.

[484] This species often takes to the water. Some individuals that I kept alive on board used to spend a quarter of an hour at a time in the water eating voraciously all the while.

[485] According to Prof. Semper, these two species in the Philippines live a large portion of the year high up on the trees in mangrove swamps. (Ibid.)

I have already referred to the circ.u.mstance that in the higher portions of the St. Christoval streams, where the rocks are entirely volcanic, the fresh-water sh.e.l.ls--and I may here add, especially those of _Neritina subsulcata_--suffer much more erosion than do sh.e.l.ls of the same species in the lower parts of the streams where they flow through calcareous districts. Now, the geological structure of this island being mainly ancient volcanic rocks incrusted near the coast by recent calcareous formations, the time will come when these calcareous envelopes will have been entirely stripped off by denudation. How this will influence the Nerites of the streams may be thus explained. At present the normal characters of the species are preserved in the calcareous portions of the streams; but when all the calcareous rocks have been stripped off by denudation, the Nerite through its whole lifetime will be subjected to that extensive process of erosion, which now often denudes almost the entire surface of the sh.e.l.ls of those individuals that live in the volcanic portion of the stream's course.

Here, Natural Selection may step in to favour the survival of any slight variation that makes the Nerite more suited to lead an entirely arboreal existence. Such a geological agency may in truth lead finally to the expulsion of the Nerite from the stream's course. Varieties will survive only in proportion to their capability of adapting themselves to the new condition; and they alone will perpetuate their kind until a tree Nerite of distinct specific character is produced..... On this reasoning, tree Nerites ought to be more numerous in islands of volcanic formation; but this is a point on which I cannot p.r.o.nounce from the lack of sufficient evidence.[486]

[486] Prof. Semper's observations in the Philippines bear on this matter. ("Natural Conditions of Existence," &c., p. 188.)

According to Professor Semper, we have in _Navicella_ "a modified form of _Neritina_," which genus it resembles in all essential anatomical characters, but "by long inurement to living in rus.h.i.+ng mountain streams, it has had its sh.e.l.l modified in the way most suited to those conditions, while the _operculum_, in consequence of long disuse, has become a peculiar degenerate or rudimentary organ."[487]

[487] Ibid, p. 212.

The growth of the fresh-water Nerites would appear to be slow. I kept a young individual of _Neritina subsulcata_ for seven months in a bottle partly filled with rain-water, and supplied it with decaying leaves for food which it used to eat. Its weight was 37 grains both at the beginning and the end of the experiment, having only varied half a grain during the whole time; and its dimensions, as determined by measurement, were unaltered. This species, when it is first picked off the rock, ejects a watery fluid with a powerful musky odour, which effect accompanies the closure of the sh.e.l.l by the _operculum_. I kept some individuals of this species in rain-water, containing varying proportions of lime-water, for about three months. The lime-water was of the medicinal strength of the British Pharmacopeia. I began with water containing 64 parts of rain-water to one part of the lime-solution. By the end of the first month the proportion was increased to 32 to 1; by the end of the second month it was 16 to 1; towards the end of the third month the Nerites, having lived for over three weeks in the last solution, began to die; the survivors were placed in a solution containing the proportion of 8 to 1, but this amount of the lime-solution proved too much for them. It should be remarked that throughout the experiment, the Nerites used to descend to the water to get their food just as frequently as in the state of nature: they did not avoid the water; and after the experiment was over, there was no apparent alteration in the appearance of the sh.e.l.ls. These observations were made in the north part of New Zealand during the latter part of the summer and the beginning of the autumn, a circ.u.mstance which may partially explain the death of the sh.e.l.ls. The temperature there was about 20 below the temperature they are accustomed to in the Solomon Islands; this difference is of interest when it is remembered that _Neritinae_ are mostly found in the streams of tropical regions; and I may, therefore, infer that this species is capable of adapting itself to temperatures much lower than that to which it is accustomed, since some individuals survived the voyage to New Zealand from the Solomon Islands and lived in the climate of the former region for three months under very unfavourable conditions.

Professor Semper[488] remarks that some _Neritinae_ have the habit of detaching themselves from rocks on the slightest touch, by this means, as he considers, escaping the pursuit of their enemies. Some of them, however, as I observed, detach themselves spontaneously and independently of any alarm. The individuals of _Neritina subsulcata_ that I kept in a large bottle in my cabin, used frequently in the course of a night to detach themselves from the sides and drop down into the water below. On one occasion when the noise woke me up, I found the culprit voraciously eating a portion of decayed leaf. In the daytime they sometimes dropped, and at other times crawled, down to the water.

..... It is probable that the musky water, which this Nerite ejects when it is picked off a rock, may cause a bird to drop it from its beak and thus save its life.

[488] Ibid, p. 210.

Amongst the new fresh-water sh.e.l.ls that I found in this group was a species of _Unio_, to which Mr. Smith did me the honour of attaching my name, it being the first species of this genus of river-mussels that has been found in the Solomon Group. But its occurrence there means something more than a new locality, since, as I believe, I am correct in a.s.serting, we have in it the first record of this widely distributed genus having reached the Pacific islands. I do not think that this species can be generally spread through the Solomon Group. I only found it in one locality, namely the Shortland Islands, near the western end of the group.

A very familiar sh.e.l.l, in low-lying moist and marshy situations throughout the Solomon Islands, is that of the auriculoid, _Pythia scarabaeus_, Linne. Being usually accustomed to find it in the low-lying districts, I was surprised on one occasion to find it in the higher parts of Faro Island, which attains an elevation of 1,900 feet above sea. In the mangrove swamps and in the lower parts of the streams at Choiseul Bay, I found a species of _Cyrena_ which has not yet been described, together with _Cerithidea cornea_ (A. Adams: var.) and _Pyrazus pal.u.s.tris_, the last species occurring also in India. On the moist ground of the taro patches in the islands of Bougainville Straits thrives a species (_S. simplex_, var.) of that ubiquitous genus _Succinea_. The operculated land-snails, of which the _Helicinae_ are the most numerous, are found more frequently in calcareous districts.

LIST OF LAND AND FRESH-WATER Sh.e.l.lS COLLECTED IN THE SOLOMON ISLANDS[489] DURING 1882 AND 1883. (EXTRACTED FROM MR. E. SMITH'S PAPER IN THE PROCEEDINGS OF THE ZOOLOGICAL SOCIETY OF LONDON, JUNE, 2ND, 1885.) THE DESCRIPTIONS OF THE NEW SPECIES AND VARIETIES FOLLOW:

[489] The habitats given are confined to the Solomon Group. I have added the new habitats of species in my collection to those previously ascertained from the collections of Brenchley, Macgillivray, Hombron and Jacquinot, etc.

(1) _Helicarion planospira_ (Pfeiffer) _Hab._ Santa Anna, Ugi, St.

Christoval, Guadalcanar.

(2) _Helix_ (_Nanina_) _nitidissima_ (nov. spec.) _Hab._ Treasury Island, a variety in Guadalcanar.

(3) _Helix_ (_Nanina_) _solidiuscula_ (nov. spec.) _Hab._ Santa Anna, found generally on the trunks of cocoa-nut palms.

(4) _Helix_ (_Corasia_) _tricolor_ (Pfeiffer) _Hab._ St. Christoval, Ugi, Santa Anna.

(5) _Helix_ (_Corasia_) _anadyomene_, A (Adams & Angas) _Hab._ Guadalcanar, Ugi.

(6) _Helix_ (_Geotrochus_) _acmella_ (Pfeiffer) _Hab._ Faro Island, Bougainville Straits; Florida Islands, _vide_ original paper.

(7) _Helix_ (_Geotrochus_) _gamelia_ (Angas) _Hab._ Isabel, Stephen Island, Shortland Islands, Treasury Island, Choiseul Bay.

(8) _Helix_ (_Geotrochus_) _hargreavesi_ (Angas) _Hab._ Faro Island in Bougainville Straits.

(9) _Helix_ (_Geotrochus_) _mendana_ (Angas) _Hab._ Shortland Islands, _vide_ original paper.

(10) _Helix_ (_Geotrochus_) _motacilla_ (Pfeiffer) _Hab._ Simbo or Eddystone Island, also called Narovo.

_Note._--In the original paper, Simbo and Eddystone are referred to as two different islands. This mistake arose from the omission of the name of Simbo in the latest charts; it is, however, the name usually employed.

(11) _Helix_ (_Geotrochus_) _guppyi_ (nov. spec.) _Hab._ Faro Island in Bougainville Straits.

(12) _Helix_ (_Geotrochus_) _dampieri_ (Angas) var. _Hab._ Choiseul Bay.

(13) _Helix_ (_Geotrochus_) _eros_ (Angas) _Hab._ Isabel, Stephen Island, Shortland Islands.

(14) _Helix_ (_Geotrochus_) _cleryi_ (Recluz) _Hab._ Santa Anna, Ugi, St. Christoval, Guadalcanar, Rua Sura Islets, New Georgia, Simbo or Eddystone, Treasury, Shortlands, Choiseul Bay. Three new varieties, var _meridionalis_ (Santa Anna), var _simboana_ (Simbo or Eddystone), var _septentrionalis_ (Shortlands, Treasury, Choiseul Bay.)

(15) _Helix_ (_Videna_) _merziana_ (Pfeiffer) _Hab._ St. Christoval, New Georgia, Ugi; _vide_ original paper.