FOSSIL SHELLS ILLUSTRATED BY RECENT 237 



the latter appearing, for the first time, after the total anni- 

 hilation of many species and genera of a more complex 

 character.* 



The prodigious number, variety, and beauty, of extinct 

 Chambered shells, which prevail throughout the Transition 

 and Secondary strata, render it imperative that we should 

 seek for evidence in living nature, of the character and habits 

 of the creatures by which they were formed, and of the 

 office they held in the ancient economy of the animal world. 

 Such evidence we may expect to find in those inhabitants of 

 the present sea, whose shells most nearly resemble the ex- 

 tinct fossils under consideration, namely, in the existing Nau- 

 tilus Pompilius, (See PI. 31, Fig. 1,) and Spirula, (PI. 44. 

 Figs. 1, 2.t) 



* The multiplication, in the Tertiary periods, of a class of animals of lower 

 organization, viz. the carnivorous Tracheiipods, (See Chap. XV. Section 1,) 

 to fill the place which, during the Secondary periods, had been occupied by 

 a higher order, namely, the carnivorous Cephalopods, affords an example of 

 Retrocession which seems fatal to that doctrine of regular Progression, 

 which is most insisted on by those who are unwilling to admit the repeated 

 interferences of Creative power, in adjusting the successive changes that 

 animal life has undergone. 



It will appear, on examination of the shells of fossil Nautili, that they have 

 retained through strata of all ages, their aboriginal simplicity of structure, 

 a structure which remains fundamentally the same ia the Nautilus Pompilius 

 of our existing seas, as it was in the earliest fossil species that we find in the 

 Transition strata. Mean time the cognate family of Ammonites, whose 

 shells were more elaborately constructed than those of ^Nautili, commenced 

 their existence at the same early period with them in the Transition strata, 

 and became extinct at the termination of the Secondary formations. Other 

 examples of later creations of genera and species, followed by their periodi. 

 cal and total extinction, before, or at the same time with the cessation of the 

 Ammonites, are afforded by those cognate Multilocular shells, namely, the 

 Hamite, Turrilitc, Scaphite, Baculite, and Belemnite, respecting each of 

 which I shall presently notice a few particulars. 



t I omit to mention the mope familiar shell of the Argonaula or Paper 

 Nautilus, because, not being a chambered species, it does not apply so 

 directly to my present sabject; and also, because doubts still exist whe- 



