122 THE WASATCn AND BlilDGEU FAUN^. 



River, resemble this species more nearly than any other. They display 

 similarity in the fineness and acuteness of the ridges between the fossc-B, 

 and their disposition to break into small tubercles on the distal parts of the 

 costal bones. More perfect specimens will be neccessary to decide whether 

 this species is common to the two horizons or not. 



PLASTOMENUS Cope. 



U. S. Geol. Surv. Terrs. 1872, p. G17. Ri-port Expl. Surv. W. 100th Mer., G. M. Whcolcr, iv, pt. li, p. 



47, 1877. 



The structure of the skeleton in this genus remains incompletely known 

 in spite of the abundance of specimens which I have procured in the Eocene 

 beds of the West. As already stated, it is allied to the genus Trionyx, but 

 differs in some important points in the bones of the plastron. The hyosternal 

 bones which I have seen in P. multifoveattts are generally like those of Trio- 

 )ii/x, while the hyposternals, if I have con-ectly identified them, differ mate- 

 rially. These elements are preserved in the species named, and in P. cor- 

 rugatus, and here they display a transverse width behind the inguinal region 

 more like an Emydoid than a Trionychoid genus. The inguinal border is 

 thickened, and at the bridge somewhat recurved. The inguinal buttress is 

 in all three of the species more robust and more vertically directed than in 

 Trionyx. The post-abdominal suture is closer and less digitate in the P. tri- 

 onychoides. In P. corrugatus there is a fontanelle at the supposed post-ab- 

 dominal suture, as in Trionyx, while there is no indication, of one in the P. 

 trionychoides. The hyposternals also display a more completed ossification 

 than in Trionyx, in the fullness of the borders between the internal and 

 external digitations. Thus, in P. multifoveatus, the internal border is regu- 

 larly convex, and the processes for the episternal bone scarcely project 

 beyond it. The external digital process projects more extensively, while 

 the free ends of the ribs extend little or not at all beyond the border of the 

 caraj)ace. Among the various remains from AV'yoming and New Mexico, no 

 marginal bones have been found, nor dermal scutal sutures. 



Portions of the skeletons of the species of this genus are very abund- 

 ant in the Eocene of New Mexico and Wyoming. Though one seldom 

 obtains an entire carapace or plastron, the form, size, and sculpture indicate 



