CEPHAXOPODA. 185 



not exist in very deep "water. They were probably limited to 

 a depth, of 20 or 30 fathoms at the utmost.* 



It is certain that the sexes were distinct in the tetrabranchiata. 

 M. D'Orbigny, noticing that there were two varieties of almost 

 every kind of ammonite — one compressed, the other inflated 

 ■ — naturally assumed that the first were the shells of male indi- 

 viduals (o), the second of females (9). Dr. Melville has made 

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

 the umbilicated specimens are the males, the imperforated shells, 

 females. Professor Yan der Hoeven has described the difference 

 in the shells of the two sexes ; f but these are trivial as com- 

 pared with those presented by the animals. The most marked 

 is that while the female has twelve retractile tentacles, the male 

 has only eight, while the other four tentacles are coalesced 

 together to form an organ called the spadix. 



In 1865, M. Barrande published the plates to his second 

 volume on the Cephalopods of Bohemia. We have not been 

 able to see this work : but it contains 107 plates, with figures of 

 200 species of ceiDhalopods, belonging to the genera Goniatites, 

 Nothoceras, Trochoceras, Hercoceras, LituiteSy Phragmoceras, Gom- 

 pJioceras, and Ascoceras. 



Family I. — ^Natjtilid^. 



Shell. Body-clianiber capacious. Aperture simj)le. Sutures 

 simple. Siphuncle central or internal. (Figs. 50, 51.) 



Nautilus, Breynius, 1732. 



Shell involute or discoidal, few-whorled. Siphuncle central or 

 sub -central. 



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

 species it is corrugated, like the patent iron-roofing, so remark- 

 able for its strength and lightness. (Buckland.) See PI. II., 

 Pig. 10. 



The umhilicus is small or obsolete in the typical nautili, and 

 the whorls enlarge rapidly. In the palceozoic species, the 

 whorls increase slowly, and are sometimes scarcely in contact. 

 The last air-cell is frequently shallower in proportion than the 

 rest. 



* By deep water, naturalists and dredgers seldom mean more tlian 25 fatlioms, 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 m a reversed position. (Sir F-, 

 Beaufort.) 



t Annals of Natural History, vol. xix. 1857. 



