HYDROSAURIA. 221 



Compsognathus, very long cervical vertebrae, an almost bird-like head, 

 a very long neck, short anterior and very long posterior ribs. The 

 astragalus is fused with the long tibia, as in birds. 



The Pterosauria or Pterodactyls, which likewise lived principally 

 in the Jurassic period, were flying Saurians. The external finger of 

 the hand was elongated in the form of a sabre, and of considerable 

 strength (fig. 118); it probably supported an expansion of the 

 integument, which enabled the animal to float along in the air, or 

 even to fly. 



RTiamphorhynclms Gemmingii H. v. M., lithographic slate- 

 Pterodactylus longirostris Cuv., Jura. 



Sub-class 2. HYDROSAURIA.* 



Aquatic Reptiles of considerable size, with teeth wedged into the 

 jaws, and leathery or armoured skin, with swimming jins or powerful 

 feet, the digits of which are connected by webs. 



The Hydrosaurians, represented at the present time by the Croco- 

 diles, are characterised by their usually gigantic size, by an organi- 

 sation corresponding to their aquatic habits, and by their high 

 development. 



The numerous fossil forms were exclusively inhabitants of the sea, 

 and had swimming fins resembling those of Whales ; the bones of 

 the arm were sliort, the carpal bones and the phalanges were numer- 

 ous, and the digits were connected. The vertebral column, which 

 was movable in its individual regions, and still composed of broad 

 amphicoelous vertebrae, was prolonged into a tail of considerable size, 

 which was probably surrounded by a membranous fin. At a higher 

 grade of development the vertebral column consists of opisthoccelous 

 reptilian vertebrae, and ends with a swimming-tail, surrounded by 

 a cutaneous fold. The extremities become more and more like 

 feet, and the distinctly separated digits are still webbed. Such 

 forms no longer inhabit the high sea, but are found on the coast 

 in lagoons, and near the mouths of rivers. They go up on to the 

 land, and move quickly upon it, but they are unable to turn quickly 

 and easily. 



The dentition shows that the Hydrosaurians are powerful preda- 



* R. Owen, "Pal asontology." London, 1860. 



Huxley, " On the dermal armour of Jacare and Caiman, etc." Journ. Proceed, 

 Linn, foe., vol. iv., 1860. 



Rathke, " Untersuchungen liber die Entwickelung und den Korperbau der 

 Crocodile." Braunschweig, 1866. 



