THE SKELETON IN REPTILES. THE LIMBS. 289 



pectoral girdle is also much reduced, in Ophisaurus apus the 

 ventral borders of the coracoids are widely separated. 



A scapula is always present, and is generally expanded 

 distally, but in the Chelonia the distal end is cylindrical. In 

 the Theromorpha it has an acromial process with which the 

 precoracoid articulates, and it is very large in Dinosauria. 

 In the Chelonia the scapula and precoracoid are ossified 

 continuously. Among the Pterosauria, Pteranodon has an 

 unique pectoral girdle; the scapula and coracoid are ankylosed 

 and the scapula articulates with the neural spines of several 

 ankylosed vertebrae. 



Clavicles occur in some Theromorpha such as Pariasaurus, 

 and also in the Ichthyosauria, Sauropterygia, Rhynchocephalia, 

 and most Lacertilia. They are absent in the Pterosauria, the 

 Chamaeleons among Lacertilia, the Ophidia and the Crocodilia. 

 They are wanting too in the Chelonia, unless the first pair of 

 ossifications in the plastron are to be regarded as clavicles. In 

 the Sauropterygia bones regarded as the clavicles and inter- 

 clavicle are generally well developed. The unpaired ossification 

 in the plastron of Chelonia is an interclavicle, and a repre- 

 sentative of the same bone occurs arising from the sternum in 

 Pterosauria. A well developed T-shaped interclavicle is found 

 in Ichthyosauria, Rhynchocephalia, Lacertilia, and some Thero- 

 morpha, such as Pariasaurus. 



THE LIMBS. 



In most reptiles there are two pairs of pentedactylate 

 limbs provided with claws, but in nearly all Ophidia and 

 some Lacertilia (Amphisbaena, Lialis, Anguis) the limbs have 

 entirely disappeared. In a few Ophidia such as Python traces 

 of the posterior limbs occur, and in Chirotes among the Amphis- 

 baenidae there are minute anterior limbs. The Lacertilians, 

 Chalcides (Seps) and Ophisaurus (Bipes, Pseudopus) have very 

 small posterior limbs. 



R. 19 



