THE SUBCLASS ANAPSIDA 223 



eluding the Athecae), the Pleurodira, Amphichelydia, and Triony- 

 choidea. The definitions of the Athecae and Thecophora given by 

 Hay are as follows: 



Athecae. Turtles which retain the primitive dermal armor, with 

 at least traces of the subdermal expansions connected with the ribs. 

 A single living species, Dermochelys coriacea. 



Thecophora. Turtles in which the primitive dermal armor is obso- 

 lete or abolished, the carapace formed by expansions of the ribs, 

 neural plates, and usually peripheral plates. 



Evidences of this primitive external series are found as vestiges, 

 perhaps, of various Cretaceous Thecophora, but they have been ac- 

 counted for in other ways. If however, the interpretation of the 

 characters of Eutiotosaurus, as given on a preceding page, is correct, 

 this theory is much strengthened, since the Eunotosauria have both 

 the expanded dermal ribs corresponding with the ordinary carapace 

 of the turtles, and an overlying carapace composed of rows of plates 

 like those of Dermochelys. 



A. Suborder Amphichelydia 



Mesoplastra present. Nasals and lacrimals distinct. Skull wholly 

 Toofed over. Neck short, not retractile, the cervical vertebrae am- 

 phicoelous or concavo-convex. 



Family Proganochelydidae. Vomer and parasphenoid with 

 small teeth. Nine dorsal ribs (costals) and seven cervical vertebrae, 

 with ribs. Neck with free dorsal plates. Scapular girdle with short 

 proscapular process or acromion; coracoid short, more or less fused 

 with scapula; a supracoracoid foramen. One sacral vertebra. (Jae- 

 kel.) 



Upper Trias. Stegochelys Jaekel,^ Proterochersis Fraas, Progano- 



chelys Baur, ? Chelyzoum Meyer, Germany. 



Family Pleurosternidae. Cervical vertebrae amphicoelous. 

 Skull elongated; coracoid distally expanded. Carapace united to 

 plastron by narrow buttresses. 



Upper Jura. Platychelys Wagler, Pleurosternum Owen, Europe. 



Cretaceous. Glyptops Marsh, North America. Helochelys Meyer, 

 Germany. 



^ [ = Triassochelys Jaekel. — Ed.] 



