12 INTERNAL SHELL. 



nucleus is observed, iudicated by the more globose first chamber, 

 as in Spirula and Belemnites. It is amongst these latter shells 

 that we find considerable modifications arising from age, sex or 

 pathological causes. The changes resulting from age are, above 

 all, visible in the rostra of the Belemnites, which, ordinarily 

 slender when j^oung, are thickened and shortened with advancing 

 age. In exceptional cases, these rostra, when their growth is 

 completed, present, at their extremity, very remarkable tubular 

 prolongations. Modifications due to sex, are shown in the dif- 

 ference in width of the shell in Loligo, in the more or less 

 elongated rostrum of Belemnites, perhaps, or in the prolongations 

 of which we have just spoken. Pathological modifications are very 

 numerous, above all in Belenniites. They may change entirely 

 the form of the rostrum, by rendering it obtuse, or even cause 

 those strange mutilations \ipon which the genus Actinocamax is 

 founded. , 



The Spirula, tlie sole survivor of a large grouit of interunl con- 

 camerated shells, is peculiar in being formed exclusively of pearl 

 (the Nautilus has an internal pearly layer) ; it hangs free in the 

 hinder end of the body, held in place solely by lateral thin 

 lappets of skin proceeding from either side of the mantle, and 

 connate below the whorls, with a in'ominence at their Junction. 

 A small portion of the intestinal sack occupies the last chamber 

 of the shell, and a i)rolongation of it connects the chambers by 

 passing through the siijhonal tubes whicli penetrate the septa* 

 towards their inner margin (instead of in the middle, as in 

 Nautilus). 



In the fossil Belemnites, the siphunculated, chambered portion 

 of the shell has been called the Phragmoconus, hy Owen ; the 

 horny or chalky blade is termed, by Huxley, the pj'o-ostracum, 

 and the ?-o.s/ry/mof the latter author" corresponds with tlie similar 

 term heretofore used by us. 



Analysis sliows the horny shell to be principally composed of 

 fiiilin. 'IMie Sepia ojfirinalis^ according to J. F. John, yields of 



Carbonate of Lime, with a trace of IMiosphate. . 8.5° 



Water, 4° 



Organic matter, 4'^ 



I'f'sidnnm, Magnesia. (Mc. . . . . .7'^. 



