TESTUDINATA. 125 



Both the hyosternal and hyposternal bones are more convex than in 

 any of the Triomjclies of the Eocene period. The ing'uinal edge is thinned, 

 and is very gently concave. The hyosternal is much thicker on the outer 

 part of its posterior suture than at the internal part Of course the same 

 is true of the hyposternal. The latter is characterized by the steepness of 

 the ascent of its external buttress, which is also situated neai-er to the 

 hyosternal suture than in Trionyches generally. The external sculptured 

 layer rises on its base and forms a narrow rim below the inguinal edge of 

 the posterior part of the hyposternal for a short distance only. The pits of 

 the inferior surface of the hyosternal are more or less parallel with the 

 borders of that bone, while those of the hyposternal are irregular. 



In a costal bone of a large specimen, the sculpture is a shallow, but 

 sharply impressed honeycomb pitting, smaller than in the preceding species 

 Thus there are seventeen or eighteen pits across the middle, to seven or 

 eight in P. trionychoides. No ribs whatever 



Measurements. 



M. 



Width of costal at middle 240 



Width of costal at end 350 



Thickness at middle 0035 



I included the specimens of this species in my descriptions of P. tho- 

 masiin the Annual Report of the United States Geological Survey for 1872. 

 When I stated there that P. tJiomasi is the type of the genus Plastomenus, I 

 referred to these specimens; it is therefore to be observed that the type of 

 this genus is really the P. multifoveatiis The true P. thomasi was founded on. 

 sternal bone perhaps of a small species of Trionyx. 



Some of the specimens of this species were found on Cottonwood 

 Creek, Wyoming, and others on the Upper Green River. 



Plastomenus molopinus Cope. 



Annual Keport U. S. Ceol. Surv. Terrs., Hayden, 1872, p. 602. Anostira molopina Cope. Proceed. Amer. 

 Philos. Soc, 1872, p. 461. Plaatomemis communis Cope, var. ii, Cope. Report Expl. Surv. W. 

 of 100th Mer., Lieut. G. M. Wheeler, iv, pt,,ii, p. 50. 



Plate XVII, figs. 9-14. 



This tortoise was common in the Bridger epoch in Wyoming, and also 

 in the Wasatch in New Mexico. My collection from the beds of the former 

 includes eight individuals in a fragmentary condition. I have already men- 



