178 



Lines. 



Width of eighth costal plate fore and aft at inner part 18 



Length of sixth costal plate at middle 58 



Length of seventh costal plate at middle 41 



Length of eighth costal plate at middle. - 22 



Many fragments, both of the carapace and plastron of soft-shelled turtles, 

 collected during Professor Hayden's expedition of 1870, and subsequently 

 by Drs. Carter and Corson at various localities in the vicinity of Fort Bridger, 

 appear to be referable to the same species as the above. 



A specimen consisting of the right half of a nuchal plate, with an attached 

 piece of a first costal, derived from the same locality as the specimen above 

 described, belonged to an animal about the same size. The width of the 

 scabrous portion of the nuchal plate in its complete condition was about 6§ 

 inches ; its fore and aft extent 1£ inches. The sculpturing of the surface is 

 more interrupted or broken than in the specimen specially referred to Trionyx 

 guttatus. The reticular ridges are narrower and sharper, and exhibit a dis- 

 position to rise in points at their intersection. 



A specimen consisting of an outer portion of an intermediate costal plate 

 measures 3f inches wide, and is 5 lines thick. The reticulation of its sur- 

 face is unbroken, but otherwise it resembles that of the nuchal plate just 

 described. 



Trionyx uintaensis. 



During my stay at Fort Bridger, in a trip to Dry Creek, Major R. S. 

 La Motte discovered the nearly complete carapace of a Trionyx, which he 

 presented to the Academy of Natural Sciences of Philadelphia. The speci- 

 men is represented in Fig. 1, Plate XXIX, one-half the natural size. On first 

 view I supposed it to belong to the same species as the former, but compar- 

 ison of the specimen with that of Fig. 1 of Plate IV leads to the belief that 

 it pertains to a different one. 



The carapace is about 16 \ inches long and 16 inches broad, so that its 

 proportions are. reversed from those in our living Trionyx muticus. It is 



■ 



about as convex as in the latter, and appears to have been slightly depressed 

 along the position of the vertebral plates, judging from that portion of the 

 shell back of the fifth costal plates, as in advance of this the specimen has 

 been crushed inwardly. The fore and back part of the carapace is truncated, 

 as in T. muticus. The posterior truncation, slightly sinuous, extends the 

 width of the last two pairs of costal plates. In T. guttatus the corresponding 



