12 INTERNAL SHELL. 



nucleus is observed, indicated 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 young, 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 Belemnites. They may chnnge entirely 

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

 those strange mutilations upon which the genus Actinocamax is 

 founded. 



The Spirilla, the sole survivor of a large group of internal con- 

 camerated shells, is peculiar in being formed exclusively of |>e;irl 

 (the Nautilus has an internal pearly laj'er) ; it hangs free in the 

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

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

 connate below the whorls, with a prominence at their junction. 

 A small portion of the intestinal sack occupies the last chamber 

 of the shell, and a prolongation of it connects the chambers by 

 passing through the siphonal tubes which penetrate the septa 1 

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

 Nautilus). 



In the fossil Belemnites, the siplmncnlated. chambered portion 

 of the shell has been called the Pli.rinjtnn<'<miix, by Owen ; the 

 horny or chalky blade is termed, by I Inxloy. t ho -/tro-ox/rcir um, 

 and the rostrum of the latter author corresponds with the similar 

 term heretofore used by ns. 



Analysis shows the horny shell to bo principally composed of 

 chit in. The ,SVym/ o///V///c///s. according to J. F. John, yields of 



Carbonate of Lime, with a trace of Phosphate. . Xf> 



Wafer. I 



Organic matter, . . . . . . .1 



Residuum, M amnesia, etc., . . . . 7 



