45 



an outer layer of opake testaceous substance, and an inner layer of naker. 

 The chambers are separated by transverse partitions (x. x. pi. I.), which are 

 concave towards the outlet or mouth of the shell, and are severally perforated 

 by a siphuncle (y. y. pi. 1.) through the centre of the disk. These chambers 

 regularly increase in size* to the last {z. pi. 1.), which forms a capacious porch 

 more than sufficient to contain all the soft parts of the anunal. Their relations to 

 this cavity will be best understood by a reference to pi. 1 . The ventral surface of 

 the body is applied to the concavity of the peripheral wall of the chamber, or keel 

 of the shell ; while the involuted convexity of the shell is adapted to the con- 

 cavity behind the hood, and is abutted against the ridge which rises from that 

 part. The hood, when the animal draws itself within its retreat, naturally forms 

 the analogue to an operculum, and, from its rigid texture, seems tolerably well 

 adapted to supply the want of that defence ; and in this function we have a further 

 instance of the analogy, before alluded to, which the hood of Nautilus bears to 

 the foot of a Gasteropod, though it be in a reverse position with respect to the 

 body. 



The points of attachment of the muscles to the shell are always to be seen 

 near the bottom of the dwelling-chamber, in the form of an expanded oval disk 

 on each side. A narrower impression connects the lateral disks so as to form with 

 them a sinuous circle corresponding to the horny belt which begirts the mantle ; 

 of which belt a layer is sometimes left adliering at this part to the inner surface 

 of recent and uncleaned shells. Not only, however, is this impression to be ob- 

 served in the last chamber, but it is also obvious in all the others, and renders 

 in each that moiety of the parietes next the mouth of the shell comparatively 

 dull, while the lower half retains the nakrous lusti-e : the indication of the mus- 

 cular attachment is also here occasionally rendered more evident by a layer of 

 the horny matter left adhering. From these appearances I conclude that the 

 process of advancement preparatory to the secretion of a new partition is by no 

 means gradual, but that the muscles, having detached their outer layer of tendon, 



* This increase is regular only as regards their superficial dimensions. In all the fuU-grown shells 

 that I have seen in section, the penultimate chamber is more shallow than those which immediately 

 precede it : during the period also in wliich the young Nautilus derives sustenance from the vitellus, 

 and when its growth may be supposed to be rapid, it probably advances proportionately further at each 

 epoch of adding a chamber, than subsequently ; and this may be the reason why after the fifth, seventh, 

 or eighth from the beginning, the chambers again become more shallow. 



