THE SKELETAL SYSTEM 



239 



amphibia differentiated into posterior and anterior moieties, the coracoid 

 and precoracoid. The dermal clavicle becomes closely apposed to the 

 precoracoid. The dorsal scapula 

 and suprascapula remain undivided 

 as in fishes. 



The dorsal arm of the pelvic 

 girdle, the ilium, articulates with 

 the transverse process of a single 

 sacral vertebra. In its most primi- 

 tive form in amphibia, the ventral 

 portion of the pelvic girdle re- 

 sembles that of ganoid fishes, and 

 consists of a broad cartilaginous 

 plate with which the femur 

 articulates. Centers of ossification 

 corresponding with ischium and 

 pubis arise successively in this 

 plate. The attempt of morphol- 

 ogists to discover in ganoid and 

 dipnoan fishes transitional stages 

 leading from the pelvic girdle of 

 eiasmobranchs to that of amphibia 

 has had little success. Fossil 

 remains also throw little light on 

 the problem. See Fig. 197. 



The girdles of reptiles are 

 essentially like those of amphibia. 

 In the turtle they become definitely 

 Y-shaped. The clavicle fuses with 

 the precoracoid and becomes 

 indistinguishable from it. The 

 ilium connects with two sacral 

 vertebrae. In pythons, a rudimen- 

 tary hip girdle connects with a pair _ -,r . , r r ^, j 



, . Fig. 195. — Ventral surface of Cladose- 



of rudimentary claws in the anal lache. In this fish the skeletons of both 



region. Both are useless; both go pectoral and pelvic fins are extremely 



° ' ° primitive. (After Jaeckel from Kingsley s 



to prove descent of serpents from "Comparative Anatomy of Vertebrates.") 



tetrapod ancestors. 



In mammals, the coracoid is reduced to a process of the scapula. In 

 man in addition to the coracoid process a remnant of the coracoid bone 

 survives in the coracoid ligament which extends from the coracoid 

 process to the sternum, and in which occasional pieces of cartilage 

 are found as rudiments of the coracoid. The clavicle has supplanted 



