582 LECTURE XXIII, 



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 transverse section of the spiral cone, but describes a slightly sinuous 

 outline. The shell consists of two substances ; 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 lustre : 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 nacreous 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 jVautilus 

 PompiUus 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 semi-nacreous 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 ai-e lined by a thin mem- 

 branous pellicle, thrown ofi" by the mantle when the animal was in the 

 act of advancing forwards to enlarge its shell and form a new septum. 



The internal cartilaginous skeleton of the Nautilus is confined to 

 the inferior surface of the head : no part of it extends above the 

 oesophagus. Viewed sideways, it presents a triangular form ; a por- 

 tion of the annular brain is protected by a groove on the upper sur- 

 face of the cartilage: two strong processes are continued from its 

 anterior and superior angles into the crura of the infundibulum, 

 giving origin to the chief muscles of that part. Two other thinner 

 processes are continued backwards, and curve inwards and down- 

 wards ; they give origin to the two great muscles which pass from 

 the internal to the external skeleton, or, in other words, attach the 

 animal to the shell. 



The muscular fibres of the head or oral sheath arise from the 

 whole of the anterior or outer part of the internal skeleton. The 

 muscular structure of the funnel presents a much greater develop- 

 ment than in the naked Cephalopods ; and, from its relation to those 



