AND AYES OF NORTH AMERICA. 



151 



TRIONYX, Qeoffr. 

 Costal bone transversely figured by narrow elevated ridges. 



T. LIMA. 



Costal bone with thick, low transverse ridges, which are connected by cross-ribs, 

 which leave scries of pits. 



T. PRISCUS. 



Costal bones with transverse irregular grooves proximally, which remain along the 

 suture only distally, leaving a triangular area of a shallow honey-comb pattern medially. 



Large, massive; the grooves of the sbell widens tbc intervening ridges. 



T. l'ENNATUS. 



Smaller, light; the grooves much wider tlian the fine separating ridges. 



T. BUIEI. 



Costal bones, with a shallow, coarse honey-comb pattern, tending to confluence dis- 

 tally. 



The vertebral segments shorter and wider; shell heavy, much, arched. 



T. HALOPHILUS. 



The vertebral segments of the carapace long and narrow ; shell thinner, flat. 



T. GUTTATUS. 



TRIONYX HALOPHILUS, Co V c. 



Proo. A. N. Sol., Phila., 1850, p. 12. 



Established upon numerous portions of vertebral sternal and costal plates, from near Summit Bridge, Newcastle 

 Co., Delaware, and from Camden Co., New Jersey; both from the lower bed of marl, according to Cook's explana- 

 tion of the Cretaceous period. 



This gpeoies differs from the Trionyx prisons Leldy, in the character of its sculpture. It is pitted coarsely and 

 regularly, live to six in an inch on a costal or vertebral plate. In the Tr. prisons the pits arc considerably smaller, 

 and disposed In scries across the costal plates, which are separated by ridges stronger and more elevated than those 

 that separate the pits themselves. The same arrangement is visible on a portion of a xiphisternal bone, from green 

 sand, near Petersburg, Va., in the collection of John S. Haines, member of the Academy, with portions of costal 

 plates, all referable to the Tr. prisons. The specimen described by Leidy was from Monmouth Co., N. J. The con- 

 tinence of several of tho pits produces a somewhat similar appearance, as is common among Trionyclics, on the outer 

 margin of the second costal plate from tho front on the left side, and probably on the extremities of the other plates, 

 mine of which are preserved. On a portion of a sternal plate, probably xiphisternal, the pits are equal and regularly 

 distributed, but much smaller than on a costal, to tho number of ton in an inch. 



One costal bears a portion of the head of the rib; one vertebral a nearly complete centrum and neural arch, and 

 another, plourapopliyses. Of a perfect vertebral plate the length and width are equal; the anterior suture concave, 

 tho posterior convex. The costal segments are markedly curved. The following measurements will furnish further 

 characters. 



