PALEOZOIC ROCKS AND ERA. 



293 



huge fish, sometimes eighteen to twenty feet long, very 

 abundant in the Devonian of Ohio. Like the Coccosteus, 

 the anterior tail was covered with broad, bony plates. 



FIG. 210. 



FIG. 211. 



FIGS. 210, 211. Devonian fishes Placoderms : 210. Coccosteus decipiens. (After 

 Owen.) 211. Diuichthys. (After Dean.) 



The Osteolepis (bony scale, Fig. 212) was covered with a 

 complete coat-of-mail of rhomboidal bony scales, like the 

 gar-fish and polypterus (Fig. 217) of the present day. 

 The Diplacanthus (double spine, Fig. 213) is more fish-like 

 in form, but is also covered with rhomboidal bony scales. 

 We draw attention to the shape of its tail. All these are 

 Ganoids. The sharks, on account of their cartilaginous 

 skeleton and imperfect scales, are known chiefly by their 

 bony spines and by their teeth. A restoration of a Devo- 

 nian shark from Ohio is given in Fig. 214. 



By examination of the figures, it is seen that Devonian 

 Ganoids are, some of them, wholly or partly covered with 

 large, immovable, bony plates (Figs. 208-211) ; others 

 with smaller, rhomboidal, bony scales (Figs. 212, 213). 

 The former are called Placo-ganoids (plate-ganoids), or 



