540 



FOSSIL TURTLES OF NORTH AMERICA. 



this species no plastral bones are mentioned, and in subsequent and more detailed descriptions 

 the plastral bones mentioned are not Tnonyx-hke, we must regard the name P. thomasii as 

 belonging to those specimens which were afterward called P. rnultifoveatus. In the Cope 

 collection of reptiles in the American Museum of Natural History there are present the median 

 ends of the right and left hyoplastra and a fragment of the left hypoplastron of a trionychid 

 which are accompanied by Cope's labels, which read as follows: "Plastomenus thomasii, 

 hyosternals" and "Plastomenus thomasii, sternum like Trionyx." The bones bear the number 

 IO22, of the American Museum. It seems quite certain that these specimens are the alleged 

 types of P. thomasii; but it is quite as certain that they have never been described. A com- 

 parison of them with the plastral bones of Bridger specimens of Platypeltis serialis proves 

 that all belong to the same species, the resemblances between the hyoplastra of No. 1022 and 

 No. 5944 being very close. 



Platypeltis amnicola Hay. 

 Fig. 697. 



Platypeltis amnicola, Hay, Bull. Amer. Mus. Nat. Hist, xxin, 1907, p. 860, pi. liv, figs. 5, 6; 

 text-fig. 19. 



The incomplete and fragmentary remains which stand as the type of the present species 

 were secured by the American Museum expedition of 1906 into the Wasatch beds of south- 

 western Wyoming. The specimen was found on Bitter Creek. There are present 2 neural 



bones, apparently enough 



FIG. 697. Platypeltis ammcola. Left fragments to restore near- 



hyoplastron and hypoplastron \ y a \\ tne costals, most of 



of type. Xi- _^ gf9 the hyoplastron and hypo- 



plastron of the left side 

 and the inner ends of 

 those of the right side. 

 The catalog number is 

 6044. 



The carapace had a 

 length of about 280 mm. 

 and a width of 200 mm. 

 or more. The species ap- 

 pears to have resembled 

 Platypeltis heteroglypta of 

 the Bridger beds. 



What is regarded as the 

 third neural is 35 mm. 

 long, 25 mm. wide, and 

 6 mm. thick. One costal bone is 46 mm. wide at the distal end, while the sixth of the left 

 side is 36 mm. wide distally. The eighth costal of the right side is present. It is small, and 

 was evidently inclosed in a notch in the seventh costal. The free ends of all the costals are 

 beveled off obliquely. 



There appear to be no welts on the costals. The pits vary considerably in size. Near the 

 distal ends the pits are arranged somewhat in rows parallel with the free border, and there are 

 usually 6, sometimes 7, in a line 20 mm. long. Toward the proximal ends the pits increase in 

 size so as to be two or three times as large as those of the distal ends. They are arranged 

 more irregularly also. 



The length of the suture between the hyoplastron and the hypoplastron (fig. 697) seems 

 to have been close to 120 mm. The bridge is 45 mm. wide, and here the bones are 13 mm. 

 thick. The lower surface of the bones, except the processes, is ornamented with ridges and 

 pits. Of the pits there are usually 5 in a line 10 mm. long. 



We may suppose that the plastron of P. heteroglypta resembled that of P. postera. That 

 of P. amnicola differs much from that of P. postera, especially in having the anterior border 

 of the hyoplastron concave, instead of nearly straight. 



