167 



teristics seems to be necessary, before an appropriate ar- 

 rangement of them can be adopted. The larger tribe has 

 been separated into twenty-two genera, all of which have 

 been found in a fossil state : whilst one genus only, nautilus, 

 is known to exist in a recent state. Two opinions are 

 entertained respecting this great disproportion between the 

 number of fossil and of recent shells of this tribe. Some 

 suppose that those genera, of which only fossil shells are 

 found, have become extinct ; whilst others believe that 

 these shells are still existing in a recent state ; but are 

 pelagian shells, their inhabitants constantly residing at the 

 bottom of the deep. This opinion is entertained by some 

 of the latest French writers, particularly by M. de Montfort. 

 . But an examination of these shells proves, that, so far 

 from their inhabitants having been destined to a constant 

 residence at the bottom of the ocean, they possessed, beyond 

 all other testaceous animals, the power of rising up to, and 

 remaining at, the surface of the sea. Supposing them, there- 

 fore, still to live, they would occasionally, as the nautilus is, 

 be seen at the surface ; but not a single instance being 

 known of a shell of these genera having been thus seen, 

 their existence may be reasonably doubted. 



The apparatus enabling the animal to raise or sink him- 

 self at pleasure is plainly discoverable in the fossil shell of 

 the nautilus: but the most important part of this organ, the 

 continuous siphuncle, is not discoverable in the dried speci- 

 mens of the recent shell. The shell in the nautilus (PI. vi. 

 fig. 1) is formed of a number, more or less, of chambers, divi- 

 ded by pierced septa. The animal resides in the largest 

 and last formed chamber ; an elastic tube, proceeding from 

 the animal, passes through the pierced septa and the several 

 chambers, and terminates in the first. Now, assuming that 

 the office of this tube is analogous with that of the swim- 

 ming bladder of fishes, it is by no means difficult to conceive 

 how the required changes of situation may be produced. 



