THE EVOLUTION OF THE REPTILES 439 



reptiles, and those forms in which the roof is complete and 

 imperforate are often grouped together as the Anapsida. 



The Chelonia are often classified among the Anapsida 

 because their skull-roof is not fenestrated, although it may be 

 reduced by emargination ; i.e. bones may be lost round the 

 edge but there is no separation of bones by a perforation 

 forming a fossa. The Permian fossil Eunotosaurus which had 

 osteoscutes and expanded ribs appears to be intermediate 

 between Cotylosauria and Chelonia. The Chelonia preserve 

 the osteoscutes (which covered the body of the Stegocephalia) 

 and they contribute to the formation of the carapace which is 

 so distinctive a feature of the Chelonia. The Triassic Triasso- 

 chelys still had teeth and the cleithrum ; these structures are 

 absent from existing forms. The clavicular pectoral girdle of 

 existing Chelonia, consisting of clavicles and interclavicle, is 

 associated with the ventral covering of osteoscutes that form 

 the plastron. The scapular pectoral girdle consists of a cora- 

 coid and a scapula bearing a large process as big as itself, 

 directed forwards and inwards. This girdle and the pelvic 

 girdle are remarkable in that they are situated within the ribs 

 instead of outside them as in normal forms. It is interesting 

 to note that some Chelonia have become secondarily adapted to 

 life in water, and their limbs have been modified into paddles 

 or flippers, as in the turtles. 



The osteoscutes of the carapace are covered by corneo- 

 scutes (" tortoise-shell ") except in Sphargis, the " leathery 

 turtle," which form is further interesting in that the carapace 

 attached to the expanded ribs as in other Chelonia is not present. 

 Instead there is a bony shell formed of a great many little poly- 

 gonal osteoscutes bearing no relation to the ribs. 



The 5th metatarsal is hook-shaped in the Chelonia, but 

 normal in the other Anapsida (Cotylosauria). 



The next group of reptiles to consider is the Synapsida. 

 They are characterised by the fact that the skull-roof is per- 

 forated by one inferior temporal vacuity or fossa, on each side, 

 and the 5th metatarsal is normal. Here belong the Plesiosaurs 

 and Theromorphs. 



The Theromorphs are a very important group. They 



