THE SHELL. 29 



amongst the extinct genera by the spirally-coiled Ammonites, 

 and other genera, and largely developed in species, but of which 

 the Nautilus is the sole recent example, the air-chambers may 

 possibly compensate the weight of superincumbent water, and 

 facilitate its crawling movements, if, as is now generally sup- 

 posed, the Nautilus is not a swimming animal, and does not 

 voluntarily^ leave its ocean-bed. The immense size and weight 

 of the Nautilus shell, capable of containing the entire animal 

 within its last chamber, the absence of long arms, or web or fins, 

 all seem to favor this supposition as to its habits.) 



3. Owing to their narrow posterior and massive anterior form, 

 as well as to the normal direction of the siphon and the frequent 

 use of the webbed arms in swimming, the cephalopods are able 

 to progress through the water more rapidly in retrograde than 

 in forward motion ; and this swimming is a succession of darts 

 made with great velocity. Here the calcareous rostrum, as in 

 the Sepia, and which is so largely developed in Belemnites and 

 other fossil genera, comes into use as a body-protector, in 

 receiA'ing and withstanding the shocks of accidental collisions. 

 It is only among the swimming species that this protection is 

 needed, and it is most required, and consequently most devel- 

 oped, in those which inhabit the vicinity of the coasts, like the 

 Sepia. 



Internal shells, having no aerial chambers, show no nucleus, 

 and do not change their forms at different periods of their 

 growth ; but in most of those furnished with the air-chambers, a 

 distinct nucleus is observed, indicated by the more globose first 

 chamber, as in Spirula and Belemnites. Amongst these latter 

 shells 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 difference 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 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 Spirula 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 prehensile prominence or 



