DEVONIAN AGE. 



263 



the Cestracion. The Corniferous of Ohio and Indiana has afforded 

 such bony pieces, showing that Cestraciont sharks were among the 

 first. For further illustration of these species, Figs. 511, 512 are 

 introduced, giving the forms of these pieces in Cestraciont sharks of a 

 later age. The Cestracion ts had teeth of nearly the usual form, at 

 the front margin of the jaw. 



Besides these Cestracionts, there were Hybodont sharks, having 

 teeth much like those of the more modern kinds of sharks. These 

 teeth occur in the American Corniferous, and have been described and 

 figured by Newberry (Ohio Geological Survey). Figs. 508, 509 rep- 

 resent those of early Mesozoic Hybodouts, while Figs. 505, 506, 507 

 give the forms of the teeth in other sharks of a still later era. 



2. Ganoids, having the body covered with shining bony scales or 

 plates, as in the Gar-pike of existing waters, and hence named Ganoid 

 by Agassiz, from ydVo?, shining. The bones of the early species were 

 cartilaginous. The scales of the ordinary Ganoids are often rhombic 

 in form (Fig. 513), and are fitted to one another like tiles; Figs. 513, 

 514, 515, 515a illustrate some of their forms and modes of junction, 

 though not drawn from Corniferous species. Fig. 570, p. 286, is a 

 foreign Devonian Ganoid, having scales of this form (Fig. 570 a). 

 Others have the bony scales nearly circular, and set on more like 

 shingles, as in the genus Holoptychius. A foreign Devonian species 

 is represented in Fig. 569, p. 286. 

 The head of a large Ganoid, found 

 in Indiana and Ohio, which New- 

 berry supposes to have had no teeth, 

 is represented, reduced, in Fig. 522. 

 Remains of a still larger species, 

 called Onychodus by Newberry, 

 occur in the Ohio Corniferous, which 

 had scales and teeth much like the 

 Holoptychius. It had jaws a foot 

 to a foot and a half long, with teeth 

 two inches or more long in the 

 lower jaw (Fig. 523), and three- 

 fourths of an inch in the upper. 

 Some of them probably had a length of twelve or fifteen feet. 



In another type of Ganoid, a bony plate covers the head, and this 



Chimxzroids. In these Squaloid groups, the spine is usually laterally compressed, and 

 if denticulate it is so along the posterior margin. In Trygon and some other genera 

 among the Rays, there is a similar spine; but it is flattened in a direction transversa 

 to the body, and has both outer edges denticulate, when either is at all so. These 

 spines in some ancient fishes were two feet or more in length (see Fig. 612, p. 308.) In 

 a living Cestracion, 23 inches long, it is 1^ in length. 



Fig. 522, Ilead of Macropetalichthys Sulli- 

 vanti (XK)\ 523, tooth of lower jaw of 

 Onychodus. 



