CHAMBERS OF NAUTILUS. 243 



times moved and fed at the bottom of deep seas, and at other 

 times rose and floated upon the surface. 



Tlie NautiU (see PI. 31. Fig. 1. and PI. 32. Figs. 1. 2.) 

 constitute a natural genus of spiral discoidal shells, divided 

 internally into a series of chambers that are separated from 

 each other by transverse plates ; these plates are perfo- 

 rated to admit the passage of a membranous tube or siphuncle 

 either through their centre, or near their internal margin. 

 (PI. 1. Fig. 31. PI. 32. Fig. 2. and PI. 33.) 



The external open chamber is very large, and forms the 

 receptable of the body of the animal. The internal close 

 chambers contain only air, and have no communication with 

 the outer chamber, excepting by one small aperture in each 

 plate for the passage of a membranous tube, which descends 

 through the entire series of plates to the innermost extremity 

 of the shell, (PI. 31, y. y. a. b. c. d. e. and PI. 32, a. b. d. e. f.) 

 These air-chambers are destined to counterbalance the weight 

 of the shell, and thereby to render the body and shell together 

 so nearly of the weight of water, that the difference arising 

 from the siphuncle being either empty, or filled with a fluid, 

 may cause the animal to swim or sink.* 



* The siphuncle represented in PI. 31, Fig. 1, illustrates the structure and 

 uses of that organ ; in the smallest whorls, from d. inwards, it is enclosed 

 by a thin calcareous covering, or sheath, of so soft a nature as to be readily 

 scraped off by the point of a quill; this sheath may admit of expansion or 

 contraction, together with the membranous tube enclosed within it. In the 

 fossil Nautili, a similar calcareous sheath is often preserved, as in PI. 32, 

 Figs. 2, 3, and PI. 33, and forms a cormected series of tubes of carbonate 

 of lime, closely fitted to the collar of each transverse plate. In four cham- 

 bers of the recent shell (PI, 31, Fig. 1, a. b, c. d.) this sheath is partially 

 removed from the desiccated membranous pipe within it, which has assumed 

 the condition of a black elastic substance, resembling the black continuous 

 siphuncular pipe that is frequently preserved in a carbonaceous state in fossil 

 Ammonites, 



At that part of each transverse plate, which is perforated for the pas- 

 sage of the siphuncle, (PI. 31, Fig, 1, y, y.,) a portion of its shelly mat- 

 ter projects inwards to about one-fourth of the distance across each cham- 



