TRIONYCHID^. 



531 



Seen from the side the cutting-edge of the maxilla is convex from front to the hinder end. 

 The zygomatic arch has only moderate width. The tympanic cavity did not extend so far 

 backward into the squamosal as it does in P. ferox and P. spinifera. 



Until we shall have secured the carapace of this species in association with the skull, we 

 must remain uncertain as to the genus to which it belongs. 



Amyda egregia sp. nov. 

 Plate 107, figs. 1-3; text-fig. 691. 



The type of this species is a complete carapace which was collected in 1895, by Mr. O. A. 

 Peterson, in charge of the American Museum expedition of that year. It was obtained from 

 the lower Washakie beds, south of Haystack Mountain, Wyoming. The number is 1186. 



The shell (plate 107, fig. i; text-fig. 691) is broad, rounded 

 in front, truncate behind, and moderately convex. The border 

 is slightly emarginate on each side at the suture between the 

 nuchal and the first costal plate; elsewhere gently repand. The 

 length of the carapace, in a straight line, is 356 mm.; the great- 

 est width is 334 mm. 



Of neural plates there are seven present. The table gives 

 their dimensions. The first 

 neural is only 25 mm. wide 

 at its hinder end. 



The nuchal has a trans- 

 verse extent of 150 mm. and measures fore and aft 22 mm. 

 Its outer extremity overlaps the free end of the rib of the 

 first costal plate. 



The shell of this species is quite thick and solid. The 

 thickness of the fifth costal plate, a little beyond the middle 

 of the length and at the sutural edge, is about 10 mm.; in 

 the middle of the width, where the rib lies, the thickness 

 equals 15 mm. 



The ribs do not appear to have projected much beyond 

 the margin of the carapace, apparently only about 30 mm. 

 At the margin the shell is beveled off rather abruptly, but 

 the sculptured layer nowhere projects beyond the deeper 

 layers of bone. On the front of the nuchal the bevel of 

 the margin becomes a broad shallow groove. 



The sculpture of this species is very coarse. In the 

 central portions of the carapace (plate 107, fig. 2) it consists 

 of rather shallow pits separated by broad low ridges, the 

 whole forming a honeycomb structure. The ridges 



are 



FIG. 691. Amyda egregia. Cara- 

 pace of type. X 6. 



nearly as broad as the pits. About three pits, occasionally four, occupy a line 20 mm. long. 

 The pitting is coarsest on the hindermost costals, and here too the ridges become more 

 elevated. On the outer fourth of the costals (plate 107, fig. 3) the ridges and pits are arranged 

 in rows across the costals, the connecting ridges being less conspicuous than those crossing 

 the plates. Near the margin the irregular honeycomb arrangement is again found. Where 

 the pits are in rows they are large and the ridges conspicuous. 



This species somewhat resembles in its sculpture Aspideretes guttatus; but in the latter 

 species the ridges separating the pits are in nearly all cases narrower and sharper. The 

 sculpture is likewise not so coarse on corresponding parts of the shell. The two species are 

 greatly different in other characters. 



Amyda umtaensis (Leidy) likewise presents a similar coarse sculpture, but the ridges 

 intervening between the pits are narrow and sharp. Other characters clearly distinguish the 

 two species. The nuchal of//, utntaensis has a considerably greater transverse extension than 

 in A. egregia. The anterior border of the shell formed by the nuchal is more truncated. The 

 neural plates differ in form and proportion, and there are only 6 of them. There are differences 

 in the last costals. 



