356 AMMONITES. 



arched, they would have been less capable of resisting pressure 

 if they had not been furnished with ribs and bosses., variously 

 disposed in the different species, for the purpose of strengthening 

 them. In the species represented in the accompanying figure, 

 these ribs are very prominent. 



FIG. 550. AMMONITES. 



898. Several kinds of chambered shells, with sinuous parti- 

 tions, are met with in various strata. These seem to have borne 

 the same relation to the Ammonite, as the Orthoceratite to the 

 Nautilus ; and they have been arranged, according to their 

 minuter diversities of structure. Some of them are spirally 

 curved, but the several whorls or turns of the spire are not in 

 contact with each other ; this is the case with the Crioceratite. 

 Others are straight, or but slightly curved ; such as the Baculite. 

 The Turrilite, again, has more of a corkscrew curve, resembling 

 that of many Gasteropod shells. There is good reason to believe 

 that this group, which most abounded at the period when the 

 Ichthyosaurus and Plesiosaurus were the principal inhabitants of 

 the sea and shores, was preyed on by those marine tyrants ; the 

 remains of the beaks, and even of the horny rings surrounding 

 the suckers, of Cephalopods, having been found in the fossilized 

 excrement of those Reptiles. 



