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perforate the mantle, and therefore cannot be directly fixed into the 

 shell ; they are, however, connected with it through the medium of 

 thin filmy layers of a corneous texture, which frequently remain at- 

 tached to the shell after the animal has been removed. The feeble 

 hold of those muscles, even in a very recent state, is thus readily ac- 

 counted for. Indeed, it is highly probable that the fixity of the body 

 of Nautilus during the inhalation and forcible ejection of the respi- 

 ratory currents is effected by the shell-muscles reacting upon one 

 another, on the principle of a spring purchase, rather than by simple 

 traction, as illustrated by the withdrawal of a gasteropod within its 

 retreat, or the closure of the valves of a conchifer by the adductor 

 muscles. 



This view, which is supported by the foregoing facts, has its princi- 

 pal basis in the line of direction of the shell-muscles, and the angle at 

 which they meet one another, at the root of the funnel-lobe ; for the 

 outer extremity of each being fixed, it follows that the first effect of 

 the contraction of the muscular fibres would be to increase the angle 

 just noticed ; and this cannot possibly be accomplished, according to 

 the recognized laws of muscular action, without tending to throw apart 

 the points of origin, or in other words, exerting outward pressure 

 against the internal wall of the shell, and thus, as it were, jamming 

 the occupant tightly in its cell. 



The action of the great lateral muscles of Nautilus here supposed, 

 affords a remarkable contrast with the mode in which the posterior 

 expanded arms of Argonauta embrace the exterior of its shell, parti- 

 cularly during the ejection of the expiratory current ; while the with- 

 drawal of the gasteropod into its abode, by the contraction of a veri- 

 table retractor, exhibits the exertion of muscular force in a very differ- 

 ent direction. 



In regard to the supposition that Nautilus macromphalus is the 

 male of N. Pompilius, I may remark, that, besides my own specimen 

 of the former, which proved to be a female, another, in very excellent 

 condition, lately deposited in the Sydney Museum, is of the same 



