12 INTKKNAI, SHKLL. 



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

 as in Spirilla and Uelemnitcs. It is amongst these latter >heils 

 that we find considerable modifications arising (Voin age. s rx or 

 pathological causes. The changes resulting tVoin age are. above 

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

 slender when young, are thickened ami shortened with advancing 

 age. In exceptional cases, these rost ra. 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 tin; more or le^ 

 elongated rostrum of Uclemnites. perhaps, or in the prolongations 

 of which we have just spoken. Pathological modifications a re very 

 numerous, above all in Belemnites. They may change 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 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 scptsc 

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

 Nautilus). 



In the fossil Uelemnites. the siphuncnlated. chambered portion 

 of the shell has been called the PfiragmoCOTlUS, by Owen: the 

 horny or chalky blade is termed, by Huxley, the i>ro-<>*l rin-n m. 

 and the /<//"/// of the latter a uthor corresponds with the similar 

 term heretofore used by us. 



Analvsis shows the horny shell to be principally composed of 

 chit in. The ^i-jn'a o///V///f///X according to .1. I'\ John, yields of 



( 'ai-bonatc of Lime, with a trace of I'liosphat e. . 85 

 Water, ......... 1 



Organic matter, . .... I 



Residuum, Magnesia, etc., . . . . 7 . 



