332 



HISTORICAL GEOLOGY. 



sizes, from half an inch to three feet in diameter. We 

 give some figures of the most characteristic forms (Figs. 

 272-374). 



It is interesting to trace the gradual changes in the 



FIG. 272. 



FIG. 273. 



FIG. 274. 



FIGS. 272-274. Jurassic cephalopoda Ammonites : 272. Ammonites margaritanus. 

 273. Ammonites Jason : side-view. 274. Ammonites cordatus : o, side-view ; 

 6, showing suture. 



form of the suture in shelled cephalopods. In the Silu- 

 rian Orthoceratites the sutures were even; in the Devonian 

 and Carboniferous Groniatites they were angled ; in the 



FIG. 276. 

 FIGS. 275, 276. 275. Belemnites Owenii. 276. Belemnites unicanaliculatus. 



Triassic Ceratites they were scalloped ; finally, here in the 

 Ammonites they were frilled in the most complex patterns. 

 Belemnites. Now, for the first time, we find the 

 highest order of cephalopods, viz., the naked ones, allied 

 to the squids, cuttle-fishes, etc. This order is represented 

 in Jurassic times by a peculiar form, called Belemnites, 



