ORDER VI CHELONIA 201 



fulhj ossißed until late in life. Plastron frequently small, cruciform, articulating with 

 the carapace hy gomphosis ; an entoplastron present. Nuchal cmargimife, with a long 

 costiform process underlying the anterior marginals. Numher of marginals eleven ; 

 a series of inframargirials. Limhs not modified into paddles ; digits moderat ely 

 elongate, webhed ; claws four or five. Upper Jura to Recent. 



Tretosternum, Owen {Pelfochelys, Dollo). Carapace with pustulate sculpture, 

 the anterior border deeply emarginate ; plastron with an intergular and five 

 pairs of plastral shields. Epidermal shields present. Purbeck and Wealden ; 

 England and Belgium. 



Platychelys, Wagner, from the Upper Jura, is sometimes referred to this 

 family. 



Toxochelys, Cope. Skull with tympanic ring open. Caudal vertebrae 

 procoelous, with well-developed chevron bones. Upper Cretaceous ; United 

 States. 



PortJwchelys, Will. Skull broad, alveolar margin very deep. Carapace 

 completely ossified, smooth, the arrangement of bones and shields nearly as 

 in Chelydra. Plastron with very small fontanelies and no sternal bridge. 

 Upper Cretaceous ; United States. 



Anostira, Leidy. Shell with vermiculated sculpture and without (or with 

 very thin) epidermal shields. Plastron articulated with the carapace by 

 suture, and bridge without distinct inguinal notch. Skull unknown. Upper 

 Eocene ; United States. 



Pseudotrionyx, Dollo. Eocene ; Belgium and England. Acherontemys, Hay. 

 Miocene ; Washington. 



Chelydra, Schweig. Shell with the emargination of the nuchal not very 

 deep; neural bones mostly hexagonal; supramarginal shields absent ; posterior 

 border of carapace serrated. Miocene to Recent. C. murchisoni, Bell. Upper 

 Miocene ; Switzerland. 



Macrodemmys, Gray (Macrochelys, Gray ; Gypochelys, Agassiz). Recent. 



Family 3. Dermatemydidae. Gray. 



Skull with open temporal fossae ; frontals not excluded from orbit ; maxilla un- 

 conneded with quadratojugal, and squamosal separated from postorhito-frontal. Shell 

 covered with epidermal shields ; plastral bones nine ; plastral shields separated from 

 the marginals by a series of inframarginals. Plastron united with the carapace by 

 suture or ligaments. Nuchal bone produced into costiform processes, underlying the 

 marginals. Number of neurals incomplete ; posterior pleurals not meeting in the 

 median line. Pubic and ischiadic symphyses widely separated. Digits moderately 

 elongate, claws four or five. Cretaceous to Recent. 



This family comprises three existing genera restricted to Central America, 

 and a number of fossil forms, such as Adocus, Agomphus {Amphiemys), and Poly- 

 thorax, Cope ; Baptemijs, Leidy ; and Trachyaspis, von Meyer. The family is 

 intermediate in position between the Chelydridae and recent Cinosternidae, 

 and in some respects suggestive of Pleurodires. Adocus, from the American 

 Cretaceous, was made by Cope the type of an independent family, character- 

 ised by the abortion of the heads of the ribs. 



The Cinosternidae, including Cinosternum {Kinosternon), Aromochelys, and 

 Goniochelys; and the Staurotypidae, including Staurotypus and Claudius, are 



