282 



HISTORICAL GEOLOGY. 



kinds are the higher. Again, now, and throughout all 

 later geological times, all the shelled cephalopods were 

 coiled, like the nautilus. But Paleozoic, and especially 

 Silurian times, were characterized by the abundance of 

 long, tapering, straight, chambered shells. These are 

 the Orthoceratites. They are entirely characteristic of 



FIG. 192. FIG. 193. FIG. 194. 



FIGS. 192-194. Silurian cephalopods : 192. Orthoceras multicameratum. (After 

 Hall.) 193. Orthoceras Duseri. (After Hall.) 194. Restoration of orthocerae, 

 the shell being supposed to be divided vertically, and only its upper part being 

 shown a, arms ; /, muscular tube (" funnel ") by which water is expelled from 

 the mantle-chamber ; <, air-chambers ; t ?, siphuncle. (After Nicholson.) 



Paleozoic, most abundant in the Silurian, and easily rec- 

 ognized by any one. We give figures of a few (Figs. 

 192-194), and an attempted restoration of the front part 

 of the shell containing the animal. 



