﻿6 BRITISH FOSSIL CEPHALOPODA. 



band, surrounding the whole body of the animal, the mantle, which is continuous 

 from one side to the other, becomes very thin and closely adherent to the shell by 

 means of a horny substance, which it secretes between itself and the shell. This 

 horny band is known as the " annulus." Nearer the dorsal side it suddenly swells 

 out into a kidney-sbaped surface, the convexity forwards, which forms the basis of 

 attachment of the great shell muscles. 



More towards the front of the body of the animal than this ring, is another circle 

 of membranous and muscular processes, which touches the first or the dorsal side 

 of the shell muscles. The dorsal part of this is a semilunar fold curved forward in 

 the centre — it lies therefore between the " hood " (to be presently described) arid 

 the dorsal fold of the mantle. On the ventral side the mantle becomes much more 

 muscular and important, forming the two sides of the funnel. From the dorsal side 

 of each muscle rises, on each side, the base of the funnel, most marked off in front, 

 where it is separated from the " head " by a furrow. The sides of the funnel 

 become longer and longer towards the ventral side, till they form two forward- 

 directed flaps (fig. 6, a), one overlapping the other, sometimes the right and some- 

 times the left, and leaving a passage between. Thus the funnel is open on the 

 ventral side. If the two flaps are unfolded, a valve is seen in the passage, arising 

 from the interval between them, and directed forwards from the inside. The 

 membrane beneath this valve is very thin, and its position is such as to prevent the 

 re-entrance of water through the funnel. The whole of the body in front of 

 this ring consists of the tentacular processes, with the oral aperture in the middle, 

 the eyes and their appendages surrounding the mouth, and, exterior to all, 

 the " hood." 



The "hood" covers the dorsal part of the "head" like the "mensum" of a 

 Natica (fig. 1, a). In shape it is conformed to the aperture of the shell (fig. 4), so 

 that it is round in front at the free side, and excavated behind (fig. 4, a) in a curve 

 parallel to the previous whorl of the shell from which it is separated by the two 

 folds above described. It is more or less produced at the sides on the hinder part, 

 so as to cover, in Nautilus pompilius, the umbilicus of the shell, which it is, perhaps 

 erroneously, credited with obliterating, in that species, by a nacreous deposit. In- 

 teriorly it is concrescent with the "oral sheath," of which it thus comes to form a 

 part ; but it is separated at the sides by a narrow groove, and the hinder angles are 

 thin, free lobes, unattached within. The exterior surface of the hood is wrinkled 

 and has numerous papillae on the sides, and in the anterior portion it is divided into 

 two parts by a median furrow, which passes to a little notch at the free border, and 

 has on either side an elevated line running longitudinally, by which it might be said 

 to be divided into three (the middle third subdivided) instead of two (fig. 4). This 

 outer surface is coloured yellow and brown, but within it is white, and has a fibrous 

 structure : it is possibly muscular. It is thickest towards its base, or hinder part. 



