38 



COMPARATIVE ANxVIOMY 



OQ either side in a laterally-directed cartilaginous projection, or 

 hasal -process. 



The relations of the arches in Elasmobranchs^ Bony Ganoids 

 and Teleosts is similar to that above described. For the further 

 streno'thening of the vertebral column so-called intercalary pieces 

 (Figs. 23, 24, 26, 28) appear between the upper and -lower arches 

 in Cartilaginous ! Ganoids and Elasmobranchs, and these in the 



Fig. 25. — Portion op the Veetelrax Columx of Protopterus. 

 C, notochord ; DF, neural spine ; FT, interspinous bone ; FS, fin-ray. 



case of the dorsal arches are often spoken of as interneural plates. 

 In Elasmobranchs the neural arch may be made up of several 

 more or less distinct pieces — the neural processes arising from the 

 centrum, the neural and interneural plates, and the neural spines. 



In the Elasmobranchii, the skeletogenous cells invade the 

 notochordal sheath, as in the Holocepliali and Dipnoi ; but the 

 sheath then becomes segmented to form a series of cartilaoiaous 



05 



Ic 



IFK. 



Fig. 26.— Portion oj? the Vertebral Colfsin of Sajmmis. 



WK, centra ; Ob, upper arches ; /<■, intercalary pieces. The apertures for^ the 

 ^spinal rerves are seen in the arclies and intercalarj' pieces. 



vertebral bodies or centra, which from the mode of their formation 

 may be called chorda-centra. The iact is thus accounted for that 

 the number of arch-elements does not necessarily correspond with 

 that of the centra iu these Fishes. Ossification may occur in the 

 concave ends of the centra and in longitudinal bars along each 

 centrum. 



