The Appendages, Anatomy, and Relationships of Trilobites Part 28

The specimen is cleaned from both upper and lower sides and, the dorsal test being removed, reveals the long blade-like setae of the exopodites, each blade being concave along its median line. They are long on the exopodites of the thoracic segments, but become shorter, without, however, any visible change of form on the pygidium. Although the posterior end is not well preserved, one gets no suggestion from a study of this side of the specimens that the exopodites of the posterior end are in any striking way different from those of segments further forward. The tips of some of the setae show minute spines, one to each blade.

On the ventral side are a number of endopodites, but they are more fragmentary than those of the other half of the specimen. Some of the exopodites are well shown, the blades being in all cases broken from the shaft. Two of the endopodites of this specimen are of especial interest, as they have interarticular membranes between the last three segments. Professor Beecher made a drawing of one of these which he placed under his pen drawing (text fig. 45).

_Measurements:_ The specimen is 5 mm. long from the front of the second thoracic segment to the end of the pygidium. The setae on the exopodites of the anterior thoracic segments are 1.7 mm. long, as exposed from the dorsal side. Some of those on the posterior part of the pygidium, only incompletely exposed, are 0.31 mm. long.

[Ill.u.s.tration: Fig. 46. _Cryptolithus tessellatus_ Green. A part of a thorax and pygidium, showing appendages. Drawn by Professor Beecher.

Specimen 238. 10.]

The dactylopodite of the first endopodite showing the articular membranes is 0.23 mm. long and 0.13 mm. wide. The propodite is of the same length and 0.17 mm. wide. The interarticular membrane between them is 0.066 mm. thick. The spines on the dactylopodite of this appendage are 0.15 mm. long. All measurements were made on photographs.

Specimen No. 238 (pl. 8, fig. 4; text fig. 46).

A triangular specimen consisting of the greater part of a pygidium and parts of all the thoracic segments. Under the thorax the specimen has been so cleaned that the outer portions of the endopodites are well shown, while under the pygidium the greater part of the endopodites seem to have been removed, disclosing the setae of the exopodites. As in other specimens, the endopodites of the thorax turn backward at the distal end of the carpopodite, which is broad and curved, and bears a tuft of spines on the posterior margin. The dactylopodites seem to preserve their natural shape, and are very nearly cylindrical in form.

Under the pygidium are several sets of overlapping fringes of setae of exopodites, and along the edge of the dorsal furrow, a number of fragments of segments of what may be c.o.xopodites while with them are a number of fragmentary shaft of exopodites.

_Measurements:_ The pygidium is 3.3 mm. long, the thorax 3 mm.

BIBLIOGRAPHY.

Aga.s.siz, L.

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Angelina N. P.

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Barth, Hermann von.

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1-3.

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Bernard, H. M.

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