2>^ 



VERTEBRATE SKELETON 



Most Elasmobranchs have short ribs articulated to the verte- 

 brae (fig. 34), and as these are in the horizontal septum, they are 

 pleural ribs, the homologues of those of Tetrapoda. In a few species 

 there are similar ribs in the tail along with normal haemal arches. 

 Ribs are lacking in ChimsDroids and many skates. 



GANOIDEI show a wide range of vertebral structure. The 

 Chondrostei (fig. 35) about parallel the Holocephali, having two sets 

 of neurals and hoemals to a somite, these resting upon the thick 



fibrous sheath of the notochord, but 

 not forming centra, the vertebrae con- 

 sisting merely of chordal sheath and 

 neur- and haemapophyses. The caudal 

 elements are best developed, forming 

 in the tail, both apophyses; in the 

 trunk the cranial parts form interca- 

 laria; the caudal, neurapophyses. The 

 haemal arch is complete with haemal 

 spine in the tail, and a haemal process 

 (p. 25) may extend between caudal 

 artery and vein. In the trunk the 

 haemapophyses are remote from each 

 other and are divided into basal 

 stumps and haemal ribs lying just 

 beneath the peritoneum. The cartilage column of sturgeons (fig. 28) 

 may have superficial ossifications on parts of the arches. Anteriorly 

 several vertebraq may fuse into a tube around the notochord, and in 

 turn, be fused with the cranium. 



Some fossil Ganoids show conditions leading to the bony Ganoids 

 of today, but some uncertainty exists as to the interpretation of 

 parts, as only the bones are preserved and there are no indications 

 of what cartilages existed. In some there was the same condition 

 (Cope's rhachltomous vertebrae) as in living Chondrostei. Others, 

 like Callopterus (fig. 20) had an unequal development of cranials and 

 caudals, these extending on the sides of the notochord so that in 

 side view it was entirely enclosed, neural and haemal elements alter- 

 nating as a series of wedge-shaped bones (embolomerous vertebrae). 

 A greater development of the cranial elements would result in centra 

 and intercentra like those of modern Aniia. 



Fig. 35. — Developing vertebrae 

 of Polyodon (Schauinsland, '05). 

 cdh, caudihaemal ; crh, cranihasmal; 

 cm, cranineural; dr, dorsal nerve 

 root; n, notochord; ns, neural spine; 

 V, intersegmental blood vessel; vr, 

 ventral nerve root. 



