168 



The weight of the shell is so counterbalanced by the empty 

 chambers, that the siphuncle passing through these cham- 

 bers, accordingly as it is dilated with gaseous or with aqueous 

 fluids, will alter the specific gravity of the whole mass, and 

 cause it either to swim or to sink. Supposing the animal to 

 be lying at the bottom of the sea, saturated with food, and 

 the siphuncle filled with a fluid ; as the food is digested and 

 decomposed, detached gas may pass from the primce vice 

 into the siphuncle, and gradually take the place of the 

 water ; when, in proportion as the specific gravity of the 

 whole mass is thus diminished, it will rise, probably into 

 that region of the waters in which the food of the animal 

 most abounds. Here, on obtaining sufficient food, or on 

 alarm from an enemy, the animal admits water into the 

 siphuncle, and immediately sinks. 



In all the other genera of this tribe, an apparatus, formed 

 of vacant chambers and a membranous siphuncle, exists, 

 capable of producing similar effects with those produced 

 by that of the nautilus ; but necessarily differing in some 

 respects, from variety of modification of the form and struc- 

 ture peculiar to each genus. The siphuncle is often very 

 well displayed in sections of the orthoceratite, and in these 

 this tube will be found to have been capable of being dilated 

 to a very considerable extent. 



Multilocular Spiral and Discoidal Shells. 



GENUS I. Nautilus. A multilocular, spiral, and sub- 

 discoidal shell ; the turns contiguous, the outer one including 

 the others ; the chambers separated by plain or nearly plain 

 transverse septa, concave outwards, and perforated by shelly 

 tubes connected by a tubular membrane so as to form a com- 

 plete siphuncle. Recent and fossil. PL vi. fig. 1. 



2. Orbulites. A multilocular, spiral, and subdiscoidal 

 shell, the turns contiguous, the outer one including the 



