TRIONYCHIDA. 531 
Seen from the side the cutting-edge of the maxilla is convex from front to the hinder end. 
The zygomatic arch has only elcrate width. The tympanic cavity did not extend so far 
Rietaed 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. 1; text- fig. 691) is broad, rounded 
l in front, truncate behind, ant moderately convex. The border 
‘i | Hi : . : 
Neural. | Length. | Width. is slightly emarginate on each side at the suture between the 
i auchal and the fact costal plate; elsewhere gently repand. The 
1 56 37 length of the carapace, in a straight line, is 356 mm.; the great- 
2 44 3! est cadena is 334 mm. 
| 3 38 31 ( ; - : 
| fi 38 BG Sf neural plates there are seven present. The table gives 
5 34 22 their dimensions. The first ee eee 
30 19 ral is only 25 vid a 
i a 28 neural is only 25 mm. wide eae 
| at its hinder end. 
J > ae 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 ols 
of the length and at the sutural edge, is about 10 mm.; in 
the ravadle of the width, where the rib hes, the Hicines 
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 
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 ne conspicuous than those crossing 
the plates. Near the margin the irregular honeytomb arrangement is again found. W here 
the pits are in rows they are large aud the ridges conspicuous. 
This species somewhat resembles i 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 uintaénsis (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 4. uintaénsis has a considerably greater transverse extension than 
in A. egregra. 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 6o0fthem. There are differences 
in the last costals. 
Fic. 691.—Amyda egregia. Cara- 
pace of type. X 4. 
