ANATOMY or NAUTILUS POMPILIUS. 37 



inch in width, in which lie the vena cava and the oviduct. Each 

 cavity has a rounded circumference, and a transverse diameter of 

 about half an inch. In a direction at right angles to this diameter 

 the dimensions vary with its state of distension ; but a quarter of 

 an inch would be a fair average. 



The anterior or outer wall of the cavity is formed by the mantle ; 

 the posterior, inner, or visceral wall by a delicate membrane. The 

 former separates it from the branchial cavity ; the latter from the 

 fifth sac, to be described by-and-by. I could find no natural aper- 

 ture in the thin inner wall, so that I conceive no communication 

 can take place between either of these sacs and the fifth sac. 



Two irregular, flattened, brownish, soft plates depend from the 

 posterior wall of the sac into its cavity ; their attached edges are 

 fixed along a line which is directed from behind obliquely forwards 

 and upwards. 



The outer and smaller of the inferior apertures on each side 

 leads into a sac of similar dimensions and constitution to the 

 preceding, but having a less rounded outline in consequence of its 

 being flattened in one direction against its fellow of the opposite 

 side, from which it is separated only by a delicate membranous 

 wall, whilst on another side it is applied against the inferior wall 

 of the superior sac, and is in like manner separated from it only 

 by a thin and membranous partition. 



Like the upper sacs, each of these has two dark-brown, lamellar, 

 glandular masses depending from its membranous visceral wall. 



A delicate, but broad, triangular membranous process, about 

 ■^th of an inch long, hangs down freely from the visceral wall of 

 the cavity just behind the opening of the short canal which con- 

 nects the sac with its aperture. 



The third and largest aperture on each side opens directly into 

 a very large fifth cavity, whose boundary is formed anteriorly by 

 the visceral walls of the sacs already described, and behind this 

 by the mantle itself as far as the horny band which marks and 

 connects the insertion of the shell-muscles. 



In fact this cavity may be said to be co-extensive with the 

 attached part of the mantle, — the viscera, enclosed within their 

 delicate " peritoneal" membranous coat, projecting into and nearly 

 filling it, but nevertheless leaving a clear space between them- 

 selves and the delicate posterior wall of the mantle. 



A layer of the " peritoneal" membrane extends from the poste- 

 rior edge of the muscular expansion which lies between the shell- 

 muscles and from the upper wall of the dilatation of the vena cava, 



