CEPHALOPODA. S3 



opening to remain tlirougli the shell, until the timhuiciis is filled up with a 

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



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

 pact ii'om the first. 



It has been stated that the sej)ta are formed periodically ; but it must not 

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

 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 

 that 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 air-cavity betw^een 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 tetrabranchiata, but since 

 only the female of the li\ang nautilus is known, we are left to conjectm*e how 

 ar the differences 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 — natm-ally assmned that the fii-st were 

 the shells of male indi^'iduals {$), the second of females ( ? ). Dr. Melville 

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

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

 is rendered probable by the circumstance, that all the knowm specimens of 

 N. pomjnlius were female, and that the supposed male {N. macromphahts) is 

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

 recent species, both the presimied sexes {N. timbilicatiis $ and N. stenom- 

 phalus $ ) are comparatively rare. 



FAMILY I. Nautilid.e. 

 Shell. Bodij-chamber capacious. Jperture simple. Sutures simple. 

 Sijjhuncle central, or internal. (Figs. 35, 36.) 



Nautilus, Breynius, 1732. 



Shell involute or discoidal, few-whirled. Siphimcle 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. {Bucklaitd.) See pi. II., 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 2651bs. 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 drawing the bottle 

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

 tion. {Sir F, Beaufort.) 



