130 THE ANATOMY OF VERTEBRATED ANIMALS. 



but existed, formerly, for a very long period of time — their 

 fossil remains occurring in rocks from the Carboniferous to the 

 older Tertiary formations, inclusively. They present curious 

 features of resemblance to the plectognath Teleostei. 



The remains of Ganoid fishes began to appear in the Upper 

 Silurian rocks at the same time as those of the JSlasmobranchii^ 

 with which they constitute the oldest Vertebrata Fauna ; they 

 al>ound in the Devonian formation, and constitute, with the 

 Jt! lasmobranchii, the whole of the Palaeozoic Fish Fauna. 

 We are in ignorance of the true affinities of Tharsis and 

 Thrissops, and of the Soplopleuridm ; but unless some, or all, 

 of these are Teleosteans, Ganoids and Elasmobranchs, alone, 

 constitute the Fish Fauna of the Mesozoic formations, as far 

 as the bottom of the Cretaceous series. 



V. The Teleostei. — The osseous fishes are occasionally de- 

 void of any exoskeleton. Sometimes they present scattered 

 dermal plates of true bone ; or, as in the Trunkfishes ( Ostra- 

 cio?i), the body may be encased in a complete cuirass, which is 

 calcified, but has not the structure of bone. Again, as in the 

 Filefishes [JBalistes), the skin may be beset with innumerable 

 small spines, somewhat like those which form the shagreen of 

 tlie Elasmobranchs in appearance, though they differ from them 

 in structure. But, usually, the exoskeleton of the Teleosteans 

 takes the form of overlapping scales, which rarely exhibit the la- 

 cunae 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 cen- 

 tra, and the primordial cartilage of the skull is more or less 

 replaced by bone. The centra of the vertebrae are usually bi- 

 concave, each face presenting a deep conical hollow. In cer- 

 tain Eels [Symbrcmchus), the centra of most of the vertebrae 

 are flat in front and concave behind, the most anterior pos- 

 sessing a convexity in front. In many Siluroid fishes a cer- 

 tain number of the anterior vertebrae are anohylosed together, 

 and with the skull, into one mass, as in the Ganoids. 



The vertebrae are distinguishable only into those of the 

 trunk and those of the tail. The latter are provided with com- 

 plete inferior arches traversed by the caudal 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-abdominal viscera. The ver- 



