Page:The Eurypterida of New York Volume 1.pdf/179

 Another interesting specimen is that reproduced on plate 5, figure 7 which shows two circular, transversely wrinkled patches behind the frontal margin.

Specimens plate 5, figures 3–5, show a trilobation of the abdomen on account of the distinct depression of the epimera. It will be noticed that the spines on the posterior margins of the tergites are restricted to the axis. Where the body is slightly bent, as in specimens figures 3 and 4, peculiar oblique folds appear, which indicate the relative thinness of this part of the integument and may correspond to folds that formed during the life of the animal. This condition gives the test a peculiar trilobitelike appearance.

We have already mentioned among the differential characters of  and   the double series of scales on the dorsal side of the postabdomen. In specimens where the inside of this integument is exposed, it is seen that between these rows the carbonaceous film appears lighter, suggesting a somewhat thinner test, which at the same time projects a little. This fact indicates that the interspace marks the position of the alimentary canal, the scales being the attachment places of the suspensory muscle strands. The interspace would then correspond to the projecting preanal ridge on the last segment in Pterygotus, which is also attributed to the pressure of the intestine.

Another specimen worthy of notice is one in which the test of the carapace is partially weathered, leaving the cast of the interior surface of the frontal part exposed and exhibiting a faint radiating structure that is restricted to the region where, from analogy with Limulus, the liver must have been located. It resembles strikingly the supposed liver impressions seen in some trilobites.

Harlan. Geol. Soc. Pennsylvania Trans. 1834. 1: 98, pl.5

Hibbert. Roy. Soc. Edinburgh, Trans. v. 13, pl. 12

Bronn & Roemer. Lethaea, 3d ed. 1854. 2: 666, pl. 91, fig. 1