G18 GEOLOGICAL SUEVEY OF THE TERRITORIES. 



toriial inclosiiig a median foutauelle and uniting by broad suture with 

 a clavicle, (episterual.) 



This genus is highly interesting as connecting more or less nearly the 

 genus Trionyx with the Chelydrine form Auostira. It is represented by 

 several species in the Bridger Eocene, all of which have the sciil])ture of 

 both of the genera named. The plastron is ossified nearly as in A iw.s- 

 tira, but in the numerous specimens obtained there was not one mar- 

 ginal bone. Nevertheless the strong emargination of the proximal euol 

 of the second costals proves the presence of a nuchal marginal, which 

 does not exist in Trionyx ; if there were other marginals they must have 

 been small and inclosed in a cartilaginous margin. The first costals 

 were much shorter than the second and much as in Trionyx. A costal 

 process of the first dorsal extended backward and was attached by 

 suture to the second costal bone just in front of its capitulum as in 

 Trionyx, Chelydra, &c. A singular sternal bone accomi)anies the speci- 

 mens of P. thomasii and P. trionychoidcs, but partially fractured in 

 both cases so as to leave its position uncertain. It can be nothing else 

 than the median portion of a h^osternal with the outer extremity 

 wanting. It bounds a fontanelle interiorly, which nearly reaches the 

 hyposternal ; anteriorly it has sutures for both mososteruum and clavi- 

 cle. It is entirely unlike anything in Trionyx ; it is thickened toward 

 the median line, and strongly sculptured externally. The hyosterual or 

 hyposternal of the " bridge "indicates that portion to have been long, and 

 about as wide as is usual in Trionyx. Its free edge is thin^ the sutural 

 union with the other component bone complete. 



The type of the genus is P. thomcmi, {Trionyx thomasii, Cope.) Other 

 species have been referred by me to Auostira. The largest S})ecies 

 (P. multifovcatus) was about as large as the snapper; the smallest (P. 

 molopinus) as large as Ghrysemys picta. 



Plastomenus thomasii, Cope. 



Trionyx thomasii, Cope. Proceed. Amer. Phil. Soc. 1872; 462. (Published July 29.) 



Eepresented by various parts of three individnals, a sufficient num- 

 ber of identical pieces being present in all to insure their specific unity. 



The bones of both carapace and plastron have a honeycomb pattern 

 of reticulation, with shallow pits, which on weathering become punctie. 

 The intervening ribs tend to connect into ridges running diagonally 

 across the costal bones. The pits tend to form linear series parallel to 

 the borders on some of the bones of the plastron. Tiie latter are flat 

 at the transveise suture. The last costal is very wide and in contact 

 with its fellow on the median line, as in other species of the genus, 

 except a sutural emargination behind, apparently for a pygal bone. 

 The outer border is straight, truncating the last rib extremity. 



Measurements. 



M. 



Thickness of a costal 004 



Width lust costal distally 048 



Width hyosterual ' U18 



Thickness hyosternal , ■ .005 



Length of a vertebral 018 



Width of a vertebral , 014 



Two of the specimens from the Bad Lauds of Cottonwood Creek, Wy- 

 oming. Named for Dr. Jos. Thomas, of Philadelphia, my former tutor. 



