10 ! \TKKN \l. SHKLL. 



the envelopment <>!' 1 he sp;>iia'.v ma--s. In anot her uToup of fossil 

 forms. iln- long shell is composed of n H;IITO\V or broad anterior 

 corm-oiis portion, and :i posterior calcareous p;irt containing the 

 a i rial chambers, placed one upon another and siphunculated. 

 These rhamlicrs are only covered with shell in (_'oin>t( nt his ; hut 

 they are protected in the Belemnites by :i testaceous ro>trum. 

 Mum-limes very lonr. which, absolutely identical with that of 

 Sepia, is composed of successive very compact radiating l:iyer>. 



'Pile study of the shell is of uTeat zoological importance. a> 

 its form mid composition vary characteristically in the dim-rent 

 genera ; and it becomes still more important ireolo^icall\ , 

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

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

 of it in comparison with recent specie.-, we are enabled not only 

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

 even to predicate the external appearance, tin- physiology, the 

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

 vertebrate paheontoloirist reconstructs a mammal or a reptile 

 from 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, are 

 threefold: 



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

 a support to the flesh, fulfilling the otlice of the -Ueleton in 

 mammals. 



2. When it is corneous or tcsl:iecoii-. and containing part-- 

 tilled with air. as in the alveola of the Belemnites. it additionally 

 represents amoiiu mollusks the swimming bladder of lishe-. 

 These air-chambers may consist, as \ve have seen, of an obliipie 

 series, separated in their interior 1>\ a ci'owd of small diaphragm-, 

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

 cuttle-bone, as in Sepia : or even of a series of chambers taking 

 a definite spiral form. as in Spirula. D'Orbiguy shows that sheila 

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

 ficiently liirht to float Upon the surface of the waves, and that 

 there is a constant coincidence <>1' the progressive augmentation 

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



