38 



COMPARATIVE ANATOMY 



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

 basal process. 



The relations of the arches in Elasmobranchs, Bony Ganoids 

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

 strengthening of the vertebral column so-called intercalary pieces 

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

 in Cartilaginous ! Ganoids and Elasmobraiichs, and these in the 



C 



FIG. 25. PORTION or THE VERTEBRAL COLUMN OF Protopterus. 

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



case of the dorsal arches are often spoken of as intcrneural 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 Holocephali and Dipnoi ; but the 

 sheath then becomes segmented to form a series of cartilaginous 



Ob 



im. 



FIG. 26. PORTION OF THE VERTEBRAL COLUMN OF Scymnw. 



WK, centra ; Ob, upper arches ; It; intercalary pieces. The apertures for^ the 

 spinal rerves are seen in the arches and intercalary pieces. 



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

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

 the number of arch -elements does not necessarily correspond with 

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

 concave ends of the centra and in longitudinal bars along each 

 centrum. 



