CEPHALOPODA. 83 



opening to remain through the shell, until the timbilicus is filled up with a 

 callous deposit; several fossil species have always a hole through the centre. 



In the ammonites^ the nucleus is exceedingly small, and the whirls com- 

 pact from the first. 



It has heen stated that the septa are formed periodically ; hut it must not 

 be supposed that the shell-muscles ever become detached, or that the aniraiil 

 moves the distance of a chamber all at once. It is most likely that the 

 adductors grow only in front, and that a constant waste takes place behind, so 

 tliat they are always moving onward, except when a new septum is to be 

 formed ; the septa indicate periodic rests. 



The consideration of this fact, that the nautilus must so frequently have 

 an au--ca\ity between it and its shell, is alone sufficient to convince us, that the 

 chambered cephalopods could not exist in very deep water. They were pro- 

 bably limited to a depth of 20 or 30 fathoms at the utmost.* 



It is certain that the sexes were distinct in the ietrabranchiata, but since 

 only the female of the living nautilus is known, we are left to coujectm-e how 

 ar the diff'erences observable in the shells, are dependant on sex. M. D'Orbigny, 

 ha\ing noticed that there are two varieties of almost every kind of ammonite, 

 — one compressed, the other inflated — naturally assmned that the first were 

 the shells of male indi\dduals {$), the second of females ( ? ). Dr. Mehdlle 

 has made a similar suggestion with respect to the nautili ; namely, that the 

 mnbilicated specimens are the males, the imperforated shells, females. This 

 is rendered probable by the cii'cumstance, that aU the known specimens of 

 N. pomjyilius were female, and that the supposed male {N. macromphalns) is 

 very rare, as we have noticed amongst the male dihranchiata. Of the other 

 recent species, both the presumed sexes (iV. umhUicatus $ and iV. stenorn- 

 $ ) are compai-atively rare. 



FAMILY I. NAUTILIDiE. 



Shell. Bodij-cliamher capacious. Aperture simple. Sutures simple. 

 Siplmncle central, or internal. (Figs. 35, 86.) 



Nautilus, Breynius, 1732. 



Shell involute or discoidal, few-whirled. Siphiincle central. 



In the recent nautili, the shell is smooth, but in many fossil species it is 

 corrugated, like the patent iron-roofing, so remai'kable for its strength and 

 lightness. {Biicklaud.) See pi. 11., fig. 10. 



* By deep water, naturalists and dredgers seldom mean more than 25 fathoms, a 

 comparatively small depth, only found near coasts and islands. At 100 fathoms the 

 pressure exceeds 265lbs. to the square inch. Empty bottles, securely corked, and 

 sunk with weights beyond 100 fathoms, are always crushed, If filled with liquid, the 

 cork is driven in, and the liquid replaced by salt water; and in drav/ing the bottle 

 tip again, the cork is returned to the neck of the bottle, generally in a reversed posi- 

 tion. {Sir F. Beaufort.) 



