10 INTERNAL SHELL. 



the envelopment of the spongy mass. In another group of fossil 

 forms, the long shell is composed of a narrow or broad anterior 

 corneous portion, and a posterior calcareous part containing the 

 aerial chambers, placed one upon another and sipliunculated. 

 These chambers are only covered with shell in Conoteuthis ; but 

 they are protected in the Belemnites by a testaceous rostrum, 

 sometimes very long, which, absolutely identical with that of 

 Sepia, is composed of successive very compact radiating layers. 



The study of the shell is of great zoological importance, as 

 its form and composition vary characteristically in the different 

 genera; and it becomes still more important geologically, 

 inasmuch as it is almost the only portion of the vast number of 

 fossil species which has been preserved to us ; and by the study 

 of it in comparison witli recent species, we are enabled not only 

 to distinguish the species and genera of these extinct forms, but 

 even to predicate the external appearance, the physiology, the 

 anatomy of the animals, with nearly the 'accuracy with which the 

 vertebrate palaeontologist reconstructs a mammal or a reptile 

 from its osseous fragments. 



The study of the internal shell, considered us to its functions 

 in the animal economy, demands some further consideration. 

 These functions, by reason of modifications of structure,- are 

 threefold : 



1. If the internal shell is a corneous blade, it becomes simply 

 a support to the flesh, fulfilling the ollice of the skeleton in 

 mammals. 



2. When it is corneous or testaceous, and containing parts 

 filled with air, as in the alveola of the Belemnites, it additionally 

 represents among mollusks the swimming bladder of fishes. 

 These air-chambers may consist, as we have seen, of an oblique 

 series, separated in their interior by a crowd of small diaphragms. 

 tilled with air, and attached to the under side of the blade or 

 cuttle-bone, as in Sepia; or even of a series of ch.-mibers taking 

 a definite spiral form, as in Spirilla. D'Orbigny shows that shells 

 of this second division, when parted from their animals, are suf- 

 ficiently light to float upon the surface of the waves, and that 

 there is a constant coincidence of the progressive augmentation 

 of the number of air-chambers with the growth of the animal, in 



