174 REPTIL1A. 



large as Protosphargis and very similar in all respects, but with 

 the plastron a little more developed. A fine carapace, with 

 some associated limb-bones, from South Dakota, is named 

 Archelon ischyros. 



Gigantic marine turtles with extremely degenerate shell are 

 still better known from the Eocene of western Europe. They 

 agree with the modern " leathery turtle" in exhibiting a dermal 

 bony shield external to the ordinary carapace ; but the two 

 best known forms seem to have a relatively larger head. 

 Eosphargis gigas, from the Lower Eocene (London Clay) of 

 the Isle of Sheppey, possesses a median dorsal row of large 

 keeled plates broader than long, and apparently a series of 

 large marginals. Psephophorus, of which the most satisfactory 

 specimens hitherto obtained are from the Upper Eocene 

 (Oligocene) of Belgium, must have been covered both dorsally 

 and ventrally with a continuous mosaic of small polygonal bony 

 plates. The same genus seems to be represented by fragments 

 of armour in the Middle Eocene of the Hampshire Basin, the 

 Upper Eocene of Alabama, U.S.A., the Miocene of France and 

 the Vienna Basin, perhaps also by a humerus in the Pliocene of 

 Antwerp. 



Several extinct genera (e.g., Lytoloma, Argillochelys, etc.) of 

 marine or estuarine turtles, comprising chiefly small species, 

 occur in the Eocene both of Europe and America. The man- 

 dibular symphysis is remarkably long, and the vacuities in the 

 shell are comparatively small. 



True marsh- and land-tortoises also date from the Eocene, 

 the shells of typical Emydians occurring in the London Clay of 

 Sheppey, and in the Upper Eocene of the Hampshire Basin, 

 while an apparently true Testudo is met with in corresponding 

 strata in Wyoming and New Mexico, U.S.A. 



The freshwater Chelydra, confined in the existing fauna to 

 parts of North and South America, is represented by a charac- 

 teristic species in the Upper Miocene of Oeningen, Switzerland, 

 and by another in the Lower Miocene lignite of Rott, near 

 Bonn, Germany. This is a case of distribution parallel with 

 that of the ganoid fishes Lepidosteus and Amia already men- 

 tioned. 



