42 PICTURESQUE SKETCHES 



but of the habits of the animal we are not able to 

 speak with any certainty. It is not unlikely, how- 

 ever, that they were exceedingly carnivorous, and 

 supplied the place of the common tribes of univalve 

 shells of after times.* They seem, also, to have 

 preceded, in some measure, the cephalopods, and 

 may, therefore, have performed the same part as the 

 animals of this fierce and powerful group. 



Of the true Cephalopoda there are several genera 

 described from the rocks of the oldest period, and 

 they differ chiefly in the shape of the singular many- 

 chambered habitation, which is, in fact, the only 

 part left by which we can identify these animals. 

 They all bore a much greater resemblance to the 

 nautilus than the cuttle-fish, and in this respect 

 seem to exhibit the same peculiarity that has been 

 already so often alluded to, namely, the usual intro- 

 duction of groups of species possessing the lower 

 organization of their tribe in the earliest formed 

 strata of the earth. 



The Nautilus (see fig. 52), the lowest existing type 

 of the Cephalopoda, (which, however, it will be re- 

 membered, form the highest division of the Mollusca,) 

 exhibits a great advance in the construction of the 

 organs of animal life, by which it is readily dis- 

 tinguished from the ordinary inhabitants of univalve 

 shells. In the first place, this animal has a true 



* The Gasteropoda themselves were not unrepresented in the seas of 

 the earlier epoch, although they do not appear in the lowest rocks of all. 

 There are at present sixty-three silurian species known a number 

 scarcely exceeding that of the Orthoceratites from the beds of the same 

 age ; while in the proportion of individuals whose fragments are found 

 there is no comparison, the latter being far more numerous. 



