CHELONIID^. 209 



the stapedial rod. Choanae placed on the line joining the centers of the orbits; the nasal 

 passages underfloored by plates from the vomer and the palatines. Neck short, head not 

 retractile. Carapace heart-shaped, excavated in front for the neck and for the fore limbs; 

 often with costo-peripheral fontanels. Plastron loosely and ligamentously joined to the cara- 

 pace; usually with median and lateral fontanels. Entoplastron small and lance-shaped. 

 Limbs unfitted for locomotion on the land; converted into paddles for swimming; the anterior 

 the more powerful. Humerus straightened and flattened; with the head nearly in the axis 

 of the bone and with the ulnar and radial processes nearly in the plane of the distal end; 

 the radial process below and free from the head. Second to fifth fingers elongated, the phalanges 

 without condyles; and all bound together in the skin and muscles. Claws I or 2. 



At the present day the Cheloniidae are represented by probably 4 genera (Eretmochelys, 

 Colpochelys, Caretta, and Chelonia) and 5 or 6 species. Most, if not all, of these species 

 enjoy a wide distribution, being found in all tropical and subtropical seas. All are seafaring 

 animals, coming on the land only to deposit their eggs. They are mostly carnivorous in their 

 food habits, but Chelonia feeds on vegetation. 



If Allopleuron hoffmanni, of Holland, belongs to this family, as usually supposed, the 

 family history extends backward to Upper Cretaceous times. Well-preserved remains of 

 sea-turtles belonging to the divisions of the Tertiary are not common. In America we have 

 a few species which are here assigned to the family with some doubt; such as those of Per i- 

 tresius of the Upper Cretaceous of New Jersey and Georgia and Lembonax of the Eocene of 

 New Jersey. It can hardly be doubted that Syllomus crispatus of the Miocene of Virginia 

 belongs to the family; while it is probable that the species here called Procolpochelys grandteva 

 and Chelonia parvitecta are also true Cheloniidae. All of these are, however, imperfectly 

 known. In Europe the known Tertiary forms are hardly more numerous or better known. 



Allopleuron hoffmanni is greatly specialized in some respects. Especially is its carapace 

 much reduced. It is improbable that from it have been derived the modern Cheloniidae. 

 It is more likely that the latter have descended from some Upper Cretaceous ancestor not 

 distant from Toxochelys. The ancestors of Allopleuron must have become sea-turtles long 

 before the Upper Cretaceous consequently long anterior to the time when the ancestors 

 of our modern Cheloniidae could be called sea-turtles. It is therefore probable that Allo- 

 pleuron really belongs to a family distinct from the Cheloniidae. 



KEY TO GENERA HERE DESCRIBED. 



A. First costal articulating with peripherals. Shell coarsely sculptured as in some 

 Trionychidse .......................... 



A A. Costal plates articulating to peripherals and sculptured. 

 A A A. Costal bones, so far as known, not articulating with peripherals and not sculptured. 



a. Plastral bones without median digitations .......... Lembonax 



aa. Plastral bones with median digitations. 



b Some neurals in contact with three pairs of costals. Rib of first costal entering 



pit in third peripheral ..................... -, Procolpochelys 



bb Neurals in contact with two pairs of costals. Rib of first costal entering pit in 

 third peripheral.. 



Genus PERITRESIUS Cope. 



Carapace cordate, as in the Cheloniidae generally; the anterior peripherals suturally 

 joined to the costals of the first pair; the posterior peripherals joined to costals by gomphosis 

 of ribs only; a high dorsal keel; surface of carapace coarsely sculptured. 



Type: Chelone ornatus Leidy. 



In 1882 (Proc. Amer. Philos. Soc., xx, p. 144) and again in 1884 (Vert. Ten. Form. 

 West, p. 112) Cope characterized this genus as having 9 pairs of costals, 2 anterior peripherals 

 joined to the costals, and a high dorsal keel. It is now evident that he had derived his infor- 

 mation from a carapace sent to him by Dr. George Little, at one time State Geologist 

 Georgia (Amer. Naturalist, m, 1878, p. 129). 





