TRIONYCHID^I. 



547 



these fragments, a piece of a costal, was figured in the report above cited. The larger piece 

 of costal which Cope figured belongs to the Geological Survey of Canada, and the present 

 writer has been enabled to study it and compare it with the carapace to be here described. 



In the collection of the American Museum of Natural 

 History, there is the nearly complete carapace of a triony- 

 chid which is with confidence referred to Cope's species. 

 This specimen, No. 6045 (plate 113; text-fig. 704) was 

 found in the Titanotherium beds of the Hat Creek Basin, 

 Nebraska, by Dr. J. L. Wortman, of the American 

 Museum's expedition of 1894. This carapace was much 

 fractured and required much skill and patience on the 

 part of the preparator to restore it. Portions of the costals 

 of the right side are missing; but the left side is complete, 

 as well as the whole margin of the right side. It is pos- 

 sible that there were originally eight pairs of costals pres- 

 ent, but the hinder border of the costals of the seventh 

 pair appear to have a smooth edge, as if having formed 

 a portion of the free border of the shell. As in P. hctero- 

 glypta, there is a neural between anterior borders of the 

 costals of the last pair present, and there is a notch in 

 the midline behind which may have lodged a small 

 eighth neural. This region resembles much that of Pla- 

 typeltis spimfera, of North America. 



The carapace is broad, slightly concave in front, and 

 slightly notcht behind. The bones are thin and light, 



FIG. 704. Platypeltis Icucopolamica. 

 Carapace. X s- No. 6045 A.M. N.H. 



indicating an animal that lived in quiet waters. This is likewise shown by the fineness of the 

 matrix inclosing it. 



The length of the carapace is close to 325 mm.; the greatest breadth is 353 mm. The 

 shell is now quite flat, but it was originally probably moderately convex. The thickness of 

 a costal plate at the sutural border and in the middle of the length is only 5 mm. or 6 mm.; but 

 thru the rib it is 7 mm. or 8 mm. All around the border the shell is beveled off. The upper, 

 or proximal, border of the bevel is concave, so that a broad, shallow groove runs around nearly 

 the whole carapace. Nowhere does the sculptured layer overhang the deeper layers of the bone. 

 Posteriorly the shell thins out to a blunt edge. 



The ribs are very distinctly markt out on the lower side of the costal plates, and they occupy 

 at least half of the width of each costal. They appear to have projected beyond the borders of 

 the costal plates only about 15 mm. 



The nuchal bone has a fore-and-aft extent of 38 mm., a transverse extent of 212 mm. At its 

 outer border it overlaps the anterior edge of the first pair of costals. 



There are present 7 neural plates. In general, these are broader behind than in front. 

 The anterior is much larger than any of the others. There is no indication of a preneural. 

 The front end of the first neural is broadest, and its anterior outer angles bound portions of 

 2 small fontanels which lie behind the nuchal. The fifth neural is quadrilateral, the sixth is 



coffin-shaped, with the broad end forward, and the seventh is 

 hexagonal and short. The table gives the dimensions of the 

 various neural plates. 



The distal ends of the various costals display considerable 

 diversity in their widths; and in the specimen in hand those of 

 the same pair differ in some cases on the opposite sides. These 

 variations may be observed on plate 113. 



The sculpture of this species varies within narrow limits in 

 different parts of the carapace; as indeed it does in all species. 

 Here, as in other cases, it is coarser on the hinder parts of the 

 shell. In general, it agrees closely with the type specimen. It consists of squarish, roundish, 

 or oval pits, inclosed by rather sharp ridges, which are narrower than the pits themselves. 

 On the neurals and on a narrow strip on each side of these (plate 113, fig. 2), and again near 



