40 COMPARATIVE ANATOMY 



intervertelrally it remains expanded and so persists as a kind of 

 connecting- or packing-substance between contiguous centra, which 

 are consequently of a deeply biconcave or ampliiccelous form (Figs. 

 29A and 29s). 



One of the Bony Ganoids, Lepidosteus, forms a marked excep- 

 tion to other Fishes as regards its vertebral column, inasmuch a& 

 definite articulations are formed between the centra, A con- 

 cavity is formed at the hinder end of each centrum (Fig. 28), which 

 articulates with a convexity on the next vertebra behind 

 (opistJioccelous form). The notochord (except in the caudal region) 

 entirely disappears in the adult ; in the larva it is seen to be ex- 

 panded intravertebrally, and constricted intervertebrally, a condition 

 of things which appears again in the higher types as, for instance^ 



FIG. 29A. DIAGRAM SHOWING THE INTER VERTEBRAL REMAINS OF THE 

 NOTOCHORD. 



C, O 1 , expanded and constricted portions of notochord ; WK, centra ; Li, intei - 



vertebral ligaments. 



FIG. 29u. PORTION OF THE VERTEBRAL COLUMN OF A YOUNG DOGFISH 

 (Scyllium canicula). After Cartier. 



notochord ; Kn, outer, and Kn l , inner, zone of cartilage ; FK, the fibre-carti- 

 laginous mass lying between these zones, which is undergoing calcification ; 

 Li, invertebral ligament. 



in Reptiles. In a still earlier larval stage, however, the constric- 

 tions are intravertebral, as in other Fishes. 



The vertebral column of Fishes is characterised by a very 

 uniform character of its elements, so that a distinction can only be 

 seen between the trunk and caudal vertebrae. Its primitive 

 character is shown by the fact that the neural arches are usually 

 incomplete dorsally. As a rule, the closing in of the arch is 

 effected by special pieces of cartilage (comp. p. 38) and by an 

 elastic longitudinal band (Figs. 24, 28) which is always present : 

 this also applies to the haemal arches. Articular processes between 

 the arches (zyyapophyses) are usually present in Fishes which 

 possess bony vertebrae; in Rays and Chimaeroids only amongst 

 Fishes are definite articulations formed between the skull and 



