318 LECTURE XXIIl. 



The skeleton of the Nautilus consists of two parts, equally distinct 

 in their position, texture, and organic properties : the one is the 

 external chambered shell; the other is a rudimental cartilaginous 

 cranium, which sends out processes for the attachment of the prin- 

 cipal muscular masses. The shell of the Nautilus consists of an 

 elongated sub-compressed cone, convoluted in close spiral whorls, 

 upon the same plane, so as to be perfectly symmetrical. In the full- 

 grown mollusk, three fourths of the shell from the commencement no 

 longer serve to lodge the animal, but have been partitioned off, as 

 they have been progressively evacuated, into a number of chambers 

 (yfig^ 129. 5, 6), increasing regularly and gradually in size from the 

 first to the last, or to the last but one. The open chamber («, a), 

 which contains the animal, is much larger than the rest, slightly 

 curved, rounded behind and on the ventral aspect, and divided into 

 two concavities on the dorsal aspect, by the projecting involuted 

 spire. On the anterior surface may be observed two slight impressions 

 of the large lateral muscles, and that of the connecting narrow cincture, 

 and, posteriorly, the infundibular aperture of the calcareous siphon 

 is seen at the middle of the last septum. This calcareous tube extends 

 about one fourth of the way towards the succeeding septum, which, 

 with all the others, is similarly perforated and prolonged backwards 

 near their middle part. The septa, about thirty-five in number, are 

 concave towards the aperture of the shell, except below, where they 

 are convex transversely ; for their circumference does not follow 

 precisely the internal surface of the spiral cone, but describes a 

 slightly sinuous outline. The shell consists of two layers ; the outer 

 one opake, white, or stained with the characteristic red- brown stripes ; 

 the internal layer is twice as thick as the former, and of a nacreous 

 structure and aspect : the external surface of the shell is naturally 

 covered with a reddish brown or greenish epiderm or periostracum ; 

 and upon the involuted convexity of the shell, the dorsal fold of the 

 mantle deposits a thin plate of a vitreous texture, stained externally of 

 a deep black colour, which can be traced as an extremely thin ad- 

 ditional layer along the interior whorls of the shell. In the Nautilus 

 Pompilius the hole or umbilicus, at the extremities of the imaginary 

 axis round which the involutions of the shell have been made, is 

 filled up by the deposition of the semi-vitreous material : but in the 

 species or variety termed Nautilus umbilicatus, the margins of the 

 dorsal fold of the mantle are not developed to the same extent, and 

 the umbilicus continues open. The septa consist exclusively of the 

 nacreous substance : they are thinnest at their margins, which, from 

 their oblique applications to the wall of the shell, increase its thick- 

 ness at the line of contact. The chambers are lined by a thin mem- 



