TESTUDINATA. 117 



deposits, as in woven linen The cervical vertebra is without spine; it is 

 not depressed in the middle, and is without any pneumatic foramen. 



M. 



Length of cervical vertebra 0C8 



Diameter at middle OiO 



Diameter at end 035 



Diameter of caudal vertebra at ball 010 



Length of caudal vertebra 023 



Length of an ungual phalange 043 



Proximal depth of an ungual phalange 013 



Length of post-abdominal, broken 180 



Width of post-abdominal 120 



A hyposternal bone of a large Trionychoid turtle displays the charac- 

 ters of this genus in its absence of superficial sculpture, and in the decus- 

 sating bone structure of its borders and processes. It belongs to a species 

 of the size of the A. byssinus, but is so much more massive than the post-ab- 

 dominal bone of that species that I suspect that it belongs to a distinct one. 

 Its median surface presents a few faint traces of tubercular roughening. 



Occasionally^ the supei-ficial layer of the bones of the plastron of 

 species of Trionyx of this formation, are found nearly smooth, but they do not 

 display the decussating bone structure, nor the thin edges of the species of 

 Axestus It has often occurred to me that these peculiarities may be the result 

 of erosion, and that the animals possessing them should be referred to the 

 genus Trionyx. This may be the case, but there are two objections to such 

 a view. First, the middle, or more prominent parts of the bone, where the 

 attrition must be greatest, displays the characters least; second, if we imagine 

 that movements of the limbs have caused the attrition, we are met by the 

 fact that the decussating structures appear on the inner borders of the bones 

 where the limbs do not reach. 



The typical specimen was found on Black's Fork of Green River; the 

 second specimen on Upper Green River. 



TRIONYX Geoifr. 



Turtles of this genus were very abundant during the Eocene period 

 in North America. They disappeai'ed from the interior basin with the close 

 of this period and did not reappear ; but they continued on the Atlantic 

 slope, and are to-day abundant in the tributaries of the Mississippi and in 

 the streams that flow into the Gulf of Mexico. 



