1881.1 ^*° [Cope. 



One row of marginal scuta ; scuta of plastron ? not distinct 



JBydropelta * Myr. 



aa. Bridges of plastron very narrow. 



/9. Carapace smooth, not sculptured. 



Two rows of marginal scuta ; five pairs of scuta of the plastron 



Macrochelys Gray. 



One row of marginals ; five pairs on plastron Chelydra Schw. 



One row of marginals ; four pairs of scuta on plastron Claudius Cope. 



/?/?. Carapace sculptured. 

 One row of marginal scuta Anostira Leidy. 



BaENID^E. 



Cope, Annual Report U. S. Geol. Surv. Terrs., 1872 (1873), p. 621. 



Supramarginal scuta (Riitimeyer) ; no interhumerals Platychelys Myr. 



No supramarginals nor interhumeral scuta Baena Leidy. 



No supramarginals ; interhumeral scuta present Polythorax f Cope. 



Adocid^e. 



Cope, Proceedings American Philosophical Society, 1870, p. 559. 

 a. Vertebral bones and scuta normal. 

 One intergular scutum entirely separating the gulars. . . . „ . .Adoeus Cope. 



Either two intergulars, or the gulars meeting behind intergular 



Amphiemys Cope. 

 aa> Vertebral bones wedge-shaped, widening upwards ; vertebral 

 scuta not wider than the bones. 

 Elements of carpace early coossified i . .Homorhoplms Cope. 



Emydid^e. 



a. No scutal sutures. 

 Surface sculptured . ApJiolidemys Pom. 



aa. Scuta including intermarginals and two anals. 



Lobes of sternum narrow Dermatemys Gray. 



Lobes of sternum wide Agomplius Cope. 



aaa* Scuta ; two anals, no intermarginals. 



Surfaces of carapace sculptured ; plastron fixed Compsemys Leidy. 



Surfaces of carapace smooth ; plastron fixed ; recent Emydidce, and the 



genus Emys Brong. % 



Posterior lobe of plastron movable ; surface smooth. .. .Ptychogaster Pom. 



Anterior and posterior lobes of plastron movable ; surface smooth 



Dithyrostemum Pict. et Humb. 



aaaa. Scuta ; one anal, no intermarginals. 

 Carapace smooth Stylemys Leidy. 



* Eury sternum Wagn. (Palceomedusa et Aciclielys Myr. (fide Riitimeyer) is 

 nearly allied to Hydropelta.) 



t Possibly one of the Ado aides ; see Proceed. Acad. Phila., Oct., 1876. 



X Gray lias distinguished several good genera among existing species on 

 cranial characters. 



