EEPTILES. 295 



rounds the anterior part of the chest, and forms sockets for 

 lodging the round head of the arm-bones, the Jiumeri. The 

 pelvic arch is in like manner formed of three pairs of bones 

 articulated to the spine by two vertebra} : the tail is more or 

 less developed. The Saurians are distinguished from Tor- 

 toises, by the absence of a carapace ; from Serpents, by the 

 possession of four extremities, moveable eyelids, and fixed 

 jaws; and from Batrachians, by their scaly skin, and the 

 absence of the metamorphosis through which that order 

 passes in early life. A great number of fossil genera refer- 

 able to extinct families have been found. Some of these 

 present modifications of the bony skeleton which are quite 

 unique. The Saurians of the secondary period depart most 

 from living forms, and those which belong to existing genera 

 are inhumed in the tertiary strata. 



1st Family. The CEOCODILIDJE, represented by the croco- 

 dile of the Nile, (Kg. 199,) includes several extinct genera. 

 The form of the articulating surfaces of the bodies of the 

 vertebra enables us to subdivide them into those which have 

 the vertebra} concavo-convex, fii-concave, and convexo-concave. 

 To the first the concavo-convex, which have the body of the 

 vertebra} concave before and convex behind belong the 

 existing genera Crocodilus and Alligator. We find several 

 species of fossil crocodiles in the tertiary strata of Europe, 

 Asia, and America. Tine skulls and other parts of the 

 skeleton of fossil crocodiles and alligators have been obtained 

 from the crocodile bed at Hordwell. The tribe with li-concave 

 vertelrce is formed of extinct genera which lived during the 

 secondary epoch. Here we group the Teleosaurus, which 

 had the muzzle and the skull of the gavial. ^Elodon, Mys- 

 triosaurns, Macrospondylus, Gnathosaurus, MacJiceosaurus, 

 Pleurosaurus, Steneosaurus, Pelagosaurus, are all found in 

 the different stages of the oolitic period. Succhosaurus and 

 OoniopJiolis are from the wealden of England ; Phytosaurus, 

 the most ancient crocodile, is from the trias of Germany ; 

 and Pcecilopleuron is from the great oolite of Normandy. 



The group with convexo-concave vertebrae are those which 

 have an inverse disposition of the articulating surfaces of 

 these bones to what exists in living types : to this group 

 belong a species of Streptospondylus, from the Kimmeridge 

 stage of Havre, and a second from the wealden of Kent. 



