ORDER CHELONIA. 1105 



of Dermatemys, and also to have an open tympanic ring. The 

 Anostirince are typically represented by Anostira, of the Upper 

 Eocene of North America and England, and we may probably 

 include in the same group Pseudotrionyx {Apholidemys), of the 

 Middle and Lower Eocene of Europe. In these forms the shell 

 has a vermiculated sculpture, but epidermal shields appear to be 

 absent, although Dr Baur states there are traces of them in the 

 typical genus ; the plastron is well developed ; and the neural bones 

 are reduced to seven, as in Dermatemys. Anostira, which, on ac- 

 count of the presence of only ten marginal bones, Dr Baur places 

 in the Dermatemydidce (Stanrotypidce), differs from Pseudotrionyx 

 by the sculpturing of the neural bones. The skull of Psendotriotiyx, 

 originally referred by Sir R. Owen to P/atemys, agrees essentially 

 with that of Macrodemmys, although the orbits are less lateral. The 

 second subfamily, Tretosternincz, is represented by Tretostemum, 

 of the English Wealden and Purbeck, with which Peltochelys, of the 

 YVealden of Belgium, is identical. Possibly the imperfectly known 

 North American Cretaceous genus Co?npsemys may prove to be a 

 closely allied, if not identical, type. The shell is studded with 

 granular tubercles, and has epidermal shields ; while the plastron is 

 larger than in the Chelydrince, and has an intergular shield. This 

 is, however, accompanied by only five paired plastral shields, as in 

 Dermatemys, instead of the six found in Pleurostermim (fig. 10 12). 

 The nuchal bone is deeply emarginate. 



Finally, in the typical subfamily Chelydrince the shell is not 

 sculptured, and has epidermal shields ; the plastron is relatively 

 small, and generally has a median vacuity ; while there is the full 

 number of eight neural bones, of which the last articulates with a 

 suprapygal, and thus prevents any of the costals from meeting in 

 the middle line. The type genus Chelydra is represented in the 

 Puerco or Lowest Eocene of the United States, and also occurs in 

 the Upper Miocene of Switzerland, the Middle Miocene of Styria 

 (where it has been described as Che/ydropsis), and the Lower 

 Miocene of Rott, near Bonn. No fossil forms have hitherto been 

 referred to Macrodemmys, which is distinguished from Chelydra by 

 the more deeply emarginate nuchal, and the lateral orbits of the 

 skull, which is very large in proportion to the shell and limb-bones. 



Family Cinosternid/E. — With the American genus Cinosternum 

 we come to a family readily distinguished by the total absence of 

 the entoplastral element of the plastron. In the existing genus the 

 skull has open temporal fossae, the nuchal has a costiform process 

 like that of the Chelydridcs, the pelvis is of a Testudinate type, and 

 the caudal vertebrae are proccelous. The absence of the entoplastral 

 also occurs in the Swiss Eocene genus Dithyrosternum, which has 

 accordingly been placed in the same family. It is distinguished from 



