CEPHALOPODA. 



opening to remain through the shell, until the umbilicus 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 been stated that the septa 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 

 fanned ; the septa indicate periodic rests. 



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

 an air-cavity between it and its shell, is alone sufficient to convince us, that the 

 chambered cephalopoda 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 tetrabrancJuata, but since 

 only the female of the living nautilus is known, we are left to conjecture how 

 ar the differences observable in the shells, are dependant on sex. At. D'Orbigny, 

 having noticed that there are two varieties of almost every kind of ammonite. 

 one compressed, the other inflated naturally assumed that the first were 

 the shells of male individuals ( $ ), the second of females ( $ ). Dr. Melville 

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

 umbilicated specimens are the males, the iniperforated shells, females. This 

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



uiii's were female, and that the supposed male (2V. macromph< 



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

 recent species, both the presumed sexes (N. umbilical us <J and N. stenom- 

 pk'ilus ^ ) are comparatively rare. 



FAMILY I. XAVTILIP.E. 

 Shell. Body-chamber capacious. Aperture simple. Sutures simple. 



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



NAUTILUS, Breynius, 1?3;2. 



7 involute or discoidal, few-whirled. Siphuncle central. 

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

 corrugated, like the patent iron-roofing, so remarkable for its strength and 

 ctfandj See pi. II., fig. 10. 



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

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

 pressuv ~>lbs. to the square inch. Empty bottles, securely corked, and 



sunk with we-.iihts be, end 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 

 np nsrain. the cork is returned to the neck of the bottle, generally in a reversed posi- 

 tion. v j> F. Beaufort.) 



