126 THE VERTEBRATE SKELETON. 



from the diverging halves of the ventral arch, and are often 

 absent. 



In Teleostei and bony Ganoids they often have different at- 

 tachments in different parts of the body. In the tail region they 

 are not differentiated from the two halves of the ventral arch, 

 which meet in the middle line, and are prolonged into a 

 haemal spine. In the posterior trunk region they sometimes 

 form distinct processes diverging from the two halves of the 

 ventral arch ; while further forward they may shift their 

 attachment so as to arise from the dorsal side of the two 

 halves of the ventral arch and at some distance from their 

 ends, which now diverge as veritri- lateral processes. 



APPENDICULAR SKELETON. 

 PECTORAL GIRDLE. 



The simplest type of pectoral girdle is found in Elasmo- 

 branchs. It is entirely cartilaginous and consists of a curved 

 ventrally-placed rod, ending dorsally in two horn-like scapular 

 processes which are sometimes attached to the cranium or 

 vertebral column. In Rays the shoulder girdle is very large, 

 and has a distinct suprascapular portion forming a broad plate 

 attached to the neural spines of the vertebrae. There is often 

 a cup-like glenoid cavity for the articulation of the limb ; this 

 cavity is specially large in Rays and is much pierced by holes. 

 In Dipnoi the cartilaginous girdle still occurs, but on it there is 

 a deposit of membrane bone forming the clavicle, inf raclavicle, 

 and supraclavicle. These bones, which with the exception of 

 the clavicle, are unknown in higher vertebrates, are better 

 developed in Ganoids, and best of all in Teleosteans. They 

 are connected by the supra- temporal with the epi-otic and 

 opisthotic regions of the cranium. Owing to this development 

 of dermal bone, the original cartilaginous arch becomes much 

 reduced, but ossifications representing the scapula and cora- 

 coid occur in bony Ganoids and Teleosteans. 



