17G 



Beneatli, the rugosities of the marginal plates have a decidedly radiant 

 appearance. The under surface of the marginals in advance of the axillary 

 notches, and the corresponding surface of the nuchal plate, are smooth or de- 

 void of the ornate rugosities. 



The pygal plate and the contiguous marginals increase in thickness from 

 their free acute edge inwardly, so as to be wedge-shaped in section. The 

 base of the wedge, directed toward the cavity of the shell, is strongly grooved 

 in the pygal plate, and gradually less so in the contiguous marginal plates. 

 The groove contributes to the general cavity of the shell. Fig. 6 represents 

 a fore and aft section of the pygal plate, exhibiting the groove on its inner part. 



The plates of the plastron exhibit their ornate ridges arranged in a radiat- 

 ing manner, as seen in Fig. 2, but they are less prominent than those of the 

 carapace. 



The shell of the specimen of Anosteira, from which the above description 

 was taken, in its entire condition, was about 5 inches in length in the median 

 line and about 4^ inches in breadth. 



Figs. 4, 5, represent two anterior marginal [)lates, showing that the species 

 reaches a much greater size. 



TRIONYX. 



Trionyx guttatus. 



One or more species of the soft-shelled turtles ( Trionyx) are indicated by 

 an abundance of fragments of shells which have come under my notice in the 

 various collections of fossils from the Bridger beds. Anything like complete 

 shells appear to be rare, as the best preserved which has yet been submitted 

 to my examination is the portion of a carapace i-epresented in Fig. 1, Plate 

 IX. The specimen, attached to a mass of sandstone, was obtained at Church 

 Buttes, near Fort Bridger, during Professor Hayden's exploration of 1868 



The osseous carapace in its entire condition is estimated to have been 

 about a foot and a quarter in length, and, independently of the extension of 

 the free ends of the riljs, has nearly reached that breadth. The bones range 

 from three to four lines in thickness, except along the position of the costal 

 ridges and near the thinner edges. 



The carapace appears to iiave had the usual composition of seven vertebral 

 plates, and eight pairs of costal plates back of the nuchal plate. It was mod- ■ 

 erately convex, and the posterior border in the specimen is deeply scolloped. 



