272 THE VERTEBRATE SKELETON. 



includes nine plates of bone, one unpaired and four pairs ; 

 they will be referred to in connection with the ribs and pec- 

 toral girdle. 



In the Leathery Turtle (Dermochelys) the carapace and 

 plastron differ completely from those of any other living 

 form. The carapace consists of a number of polygonal ossifi- 

 cations fitting closely togethei and altogether distinct from the 

 vertebrae and ribs. The plastron is imperfectly ossified, and 

 not united with the pelvis, and the whole surface of both 

 carapace and plastron is covered with a tough leathery skin, 

 without horny shields. 



Some of the extinct Dinosauria have an enormously de- 

 veloped dermal exoskeleton. Thus in Stegosaurus and Omo- 

 saurus the dorsal surface is provided with flattened plates or 

 with spines reaching a length of upwards of two feet. In 

 Polacanthus the posterior part of the body is protected by a 

 bony shield somewhat recalling that of the little armadillo 

 Chlamydophorus. No exoskeleton is known in Ichthyosauria, 

 Sauropterygia, Pterosauria, many Dinosauria and Theromorpha, 

 and some Lacertilia, such as Chamadeon and Amphisbaena. 



TEETH. 



The teeth of reptiles are generally well developed, and in 

 the great majority of forms are simple conical structures, 

 uniform in character, generally somewhat recurved, and often 

 with serrated edges. Another common type of tooth is that 

 with a laterally compressed triangular crown provided with 

 a double cutting edge which may or may not be serrated. The 

 teeth are mainly formed of dentine, with usually an external 

 layer of enamel, and often a coating of cement on the root. 

 Vasodentine is found below the dentine in Iguanodon. The 

 teeth of reptiles never have the enamel deeply infolded, nor 

 do they have double roots. 



Teeth may be present not only on the jaw-bones, but as in 

 many Squamata, also on the palatines, pterygoids or vomers. 



