THE TELEOSTEI. 149 



(Ostmcion), the body may be incased in a complete cuirass, 

 which is calcified, but has not the structure of bone. Again, 

 as in the Filefishes [Balistes), the skin may be beset with 

 innumerable small spines, somewhat like those which form 

 the shagreen of the Elasmobranchs in appearance, though 

 they differ from them in structure. But, usually, the exo- 

 skeleton of the Teleosteans takes the form of overlapping 

 scales, which rarely exhibit the lacunse characteristic of true 

 bone. The free portions of the scales are sometimes smooth, 

 and rounded at the edge, when they are termed cycloid ; or 

 they are roughened with ridges and minute spines, when 

 they are called ctenoid. 



The spinal column always presents ossified vertebral 

 centra, and the primordial cartilage of the skull is more 

 or less replaced by bone. The centra of the vertebrae are 

 usually biconcave, each face presenting a deep conical 

 hollow. In certain Eels (Symbranchus), the centra of most 

 of the vertebrse are flat in front and concave behind, the 

 most anterior possessing a convexity in front. In many 

 Siluroid fishes a certain number of the anterior vertebras 

 are ankylosed together, and with the skuU, into one mass, 

 as in the Ganoids. 



The vertebrse are distinguishable only into those of the 

 ti-unk and those of the tail. The latter are provided with 

 complete inferior arches, traversed by the catidal artery 

 and vein. The former usually possess ribs, but these do 

 not unite with one another, nor with any sternum, in the 

 ventral median line, and they enclose the thoracico-abdomi- 

 nal viscera. The vertebra are commonly united by zyga- 

 pophyses, or oblique processes, placed above the centra ; in 

 addition to which, the lower margins of the centra are, 

 not unfrequently, imited by additional articular processes. 

 Transverse processes commonly exist, but the ribs are articu- 

 lated with the bodies of the vertebrse, or with the bases of 

 the transverse processes, not with their extremities. 



When a dorsal fin exists in the trunk, its rays are articu- 

 lated with, and supported by, elongated and pointed bones 

 — the interspinous bones, which are developed around pre- 



