12 I \TKU\AL SHELL. 



nucleus is observed, indicated by the more globose iirst chamber, 

 MS iii Spirilla .-iiid I.elemnites. It is amongst these latter shells 

 ih.-il we find consider.-! l>le modifications arising from age, sex or 

 pathological causes. The changes resulting from age are. above 

 all, visible iii the rostra of the Belemnites. which, ordinarily 

 slender when young, arc 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 \vidth of the shell in Loligo. in the more or less 

 elongated rostrum of IJelemnites. perhaps, or in the prolongations 

 of which we have just spoken. Pathological modifications arc very 

 numerous, above all in Helemnites. They may change entirely 

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

 those strange mutilations nj)on which the genus Actinocamax is 

 founded. 



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

 canierate<l 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 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- 

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

 Nautilus). 



In the fossil lidemnites. the siphunculated. chambered portion 

 of the shell has been called the Pln-(i.<jinn<'<nni*< by ( )weu ; the 

 horny or chalky blade is termed, by Huxley, the pro-ntti'iim m. 



and the rostrum of the Latter author corresponds with the similar 



term heretofore used by us. 



Analysis shows the horny shell to be principally composed of 

 chit in. The Xrju'i/ njji,-iit<ili*. according to ,1. F. ,Jolm. \ ields of 



I'arbonate of Lime, wit h a l race of IMiosphate. . sf> 



Water, ....... .( 



OrL::iiiic mutter ...... .1 



lle-idnnni. Magnesia, etc., ... . 7 . 



