618 GEOLOGICAL SURVEY OF THE TERRITOEIES. 



ternal mclosiug a median fontanelle and uniting by broad suture with 

 a clavicle, (episternal.) 



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

 genus Trionyx with the Ghelydrine form Anostira. It is represenved by 

 several species in the Bridger Eocene, all of which have the sculpture of 

 both of the genera named. The plastron is ossified nearly as in Anos- 

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

 ginal bone. Nevertheless the strong emargination of the proximal end 

 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 accompanies the speci- 

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

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

 than the median j)ortion of a hyosternal with the outer extremity 

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

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

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

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

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

 about as wide as is usual in Trioyiyx. Its free edge is thinj the sutural 

 union with the other component bone complete. 



The type of the genus is P. thomasii^ [Trionyx thomasii^ Cope.) Other 

 species have been referred by me to Anostira. The largest species 

 (P. multifoveatus) was about as large as the snapper j the smallest (P. 

 onolopinus) as large as Chrysemys picta. 



Plastomenus thomasii. Cope. 



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



Eepresented by various parts of three individuals, 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 punctce. 

 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. The latter are flat 

 at the transverse 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 last costal distally 048 



Width hyosternal - OlS 



Thickness hyosternal , 005 



Length of a vertebral 013 



Width of a vertebral 014 



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

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



