io6 



THE OSTEOLOGY OF THE REPTILES 



Dorsal Vertebrae 



(Fig. 82) 



The smallest number of dorsal vertebrae known in reptiles is 

 that of the Chelonia, invariably ten. In the chameleon lizards there 

 are as few as eleven; in the pterodactyls about twelve. In the lat- 



FiG. 82. Ophiacodon mirus Marsh (Theromorpha). Seventh to 

 twentieth vertebrae, from the side. One half natural size. 



ter order three or more of the anterior ones may be more or less 

 immovably united for the support of the pectoral arch, forming the 

 notarium. In the Chelonia they are fused throughout in the cara- 

 pace. The largest number of dorsal vertebrae in reptiles having a 

 sacrum, forty-one or forty-two, is found in Pleurosaurus , a slender, 

 aquatic Jurassic reptile. About thirty is the usual number in the 

 plesiosaurs. In terrestrial reptiles the number never exceeds twenty- 

 two or twenty-three and is usually about eighteen. In reptiles lack- 



