ix PLEURODIRA 389 



reduced or absent in the Pelomedusidae, in which the palatines 

 meet. All the Chelydidae, except Chelys, have nasal bones 

 which remain distinct from the prefrontals. The choanae lie in 

 front of the palatines, divided by the vomer when this is present, 

 but they are not roofed in ventrally. 



The ilia are solidly ankylosed in the adult with the neigh- 

 bouring costal plates, mostly with the last two pairs, sometimes 

 also with the pygal plate. The lateral processes of the pubes 

 fuse with the xiphiplastra. The ischia are also attached to the 

 same plastral elements. 



The carapace is flat and completely ossified. The nuchal 

 plate is always conspicuous, much larger than the neurals, and 

 these are often reduced by being encroached upon by the eight 

 pairs of costal plates, which then meet in the dorsal line. In 

 Sternothaerus all the eight neurals are present and form a 

 continuous row. In most of the other genera they are reduced 

 to seven, the last being squeezed out. In Rhinemys they are 

 reduced to the second, third and fourth and an isolated fifth, and 

 in Hydraspis they are all gone. The pygal plate is always, even 

 in Sternothaerus, separated from the last neural by the eighth 

 pair of costals. The marginals number 23, but in Carettochelys 

 only 21. 



The carapace is covered with horny shields, except in Caretto- 

 chelys. The nuchal is absent in the Pelomedusidae and in a 

 few Chelydidae (Elseya and a few species of Emydura). In 

 Hydromedusa the nuchal is shut in by the anterior marginals, 

 simulating a sixth neural. The plastron is composed of the 

 usual nine elements, but the Pelomedusidae possess an additional 

 pair, the meso-plastra, inserted between the hyo- and hypo-plastra. 

 The bridge is strong, connected with the carapace by suture. 

 In Sternothaerus the front lobe of the plastron is movable. The 

 intergular shield is always present ; it is terminal, forming part of 

 the front margin, except in Chelodina, where this shield, although 

 large, is shut in behind the gulars (cf. Fig. 61, 4 and 5, p. 315). 



Although the Pleurodira are a peculiarly specialised group, 

 one of the oldest Chelonian fossils known seems to belong to 

 them. Proganochelys, represented by a complete shell, nearly 

 2 feet long, has been found in the Upper Keuper Sandstone of 

 Wiirtemberg. Plesiochelys, of the Upper Jurassic of Switzerland, 

 has eight neural and three supracaudal plates, but is without the 



