10 INTERNAL S1IKU.. 



tlu- envelopment of tin- spongy mass. In ;inotlicr group of fossil 

 form-, the l<>ii<;- shell is composed of :i narrow or bn>ad .-interior 

 conn-mis portion, mid :i posterior e;ilc:ireons part containing the 

 ;ei'i;il chambers, placed one upon another :iml siphunculated. 

 These chambers are onlv covered with shell in Conotcuthis ; hut 

 they arc protected in the Belemnites by a testaceous rostrum, 

 sometimes very lon^. which, absolutely identical with that ol' 

 Sepia, is composed of successive very compact, radiating layer-. 



Tin- study of the shell is of great zoological importance. afl 

 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 

 I'o^il species which has been preserved to us; and by the study 

 of it in comparison with 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 

 verlebrate paheontologist reconstruct s a mammal or a reptile 

 l'nm its osseous fragments. 



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

 in the animal economy, demands some further consideration. 

 These functions, by reason of modifications of structure, arc 

 threefold : 



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

 a Mippnrt to the llesh. fuH'illing the ollice of the skeleton in 

 mammals. 



'J. When it is corneous or testaceous, and containing parts 

 tilled with air. as in the alveola of the Helemnites, it additionally 

 represents .-1111011^ mollusks the swimming bladder of fishes. 

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

 -erie-. separated in t heir interior by a crowd of small dia phraunis. 

 filled with air. and attached to the under side of the blade or 

 e'lttle-bone, as in Sepia ; or even of a series of chambers taking 

 a definite spiral form, as in Spirilla. D'Ofbigliy shows thai shells 

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

 ficiently liuht to tloat upon the surface of the waves, and that 

 then- is a constant coincidence of the progressive aim-mentation 

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



