Page:TransRoySocCanada 16 4 159-167.djvu/9

From Wikisource
Jump to navigation Jump to search
This page has been validated.
[BELL]
167
NEW GENUS OF CHARACEAE AND NEW MEROSTOMATA

meral portions of these segments are defined from the axial portions in some segments by narrow, curved longitudinal grooves. They end in acute projections. The succeeding and last segment is formed of a fusion of somites. Anteriorly on this last segment there are two free epimeral projections. A small hemispherical tail-like area is marked by a median triangular depression suggesting articulation with a caudal spine which has been lost. In one specimen the entire posterior segment is lacking.

The total length of the abdominal portion is 8 mm., the width across the 4th segment 7.5 mm. On account of the partial covering of matrix and overlapping of the posterior shield there is a false appearance of a marked contraction of the body at the first two segments.

In the more complete specimen the cephalic shield has suffered rotation to the left and on that side overlaps all but the outer corner of the hinder shield. On account of this distortion and the presence of minute slips within the test it is difficult to trace the precise limits of the two shields in the axial region. Fig. 14 is a drawing to show more clearly the relations. A portion of the headshield a1 has been broken off and slipped under A. There is little evidence such as transverse wrinkling to indicate any important foreshortening, so that in its natural position shield A probably overlapped somewhat onto the anterior portion of shield B. A similar overlapping is apparent in the second specimen, although the margin of the fore shield is not complete. It seems evident at least that the two shields are not fused together in the axial region.

The shell substance is thin and lacks the scale-like ornamentation of the Eurypterids.

Although the general contour of the body and of the large cephalic shield resembles other Xiphosurians there is no apparent trilobation of the body and the presence of a distinct post-cephalic shield is unique. Were it not for the presence of the double shield and the absence of trilobation Schistaspis would be very similar to Belinurus, which has an abdomen of eight segments, of which the 7th and 8th are consolidated, in addition to a long, slender telson. Euproöps has seven abdominal segments fused together, besides a short caudal spine.