﻿BEITISH FOSSIL CEPHALOPODA. 7 



When the animal is retracted the anterior part of this hood is drawn backwards, so 

 that it acts as a kind of operculum. 



The " oral sheath " and its processes have different characters in the male and 

 female, and are somewhat variable in individuals. In the female, which appears to 

 be by far the commoner animal, we may distinguish, first the outer circle : on the 

 dorsal side this is concrescent with the inside of the hood, which has, on each side of 

 its middle line within, a single aperture facing forwards, from which a single 

 tentacle may be protruded : on each side the sheath consists of a broad area which 

 is produced in the anterior direction into a variable number (17 to 19) of irregularly 

 arranged digitations. Each of these digitations has an aperture at the end — their 

 exterior surface is rough or slightly canaliculate. The one nearest the hood, that 

 is, the most dorsal one, is larger than the rest, and has its exterior formed like the 

 hood itself, from which it looks as if it had been cut off (fig. 4, c). From the 

 aperture in each of these digitations (fig. 1, c) may be protruded a tentacle (fig. 1, b). 

 These tentacles lie free in long smooth canals excavated in the substance of the 

 digitations and are only attached at the base. In shape they are trihedral, their 

 inner surface is thrown into transverse deep folds, and has a longitudinal central 

 furrow, while the outside forms the rounded angle of the triangle. In transverse 

 section they show, rather nearer their inner side, a strong sheath of cellular tissue 

 which protects the nerve, between which and the inner border is the vein and 

 artery : from this sheath transverse muscular fibres arise, which diverge and seem 

 to branch out, so as to be attached chiefly to the two outer sides, leaving the inner 

 side more completely to nervous action : longitudinal muscular fibres run between 

 the branches of the transverse, and by these muscles the protrusion and withdrawal 

 is effected. The separation between the digitations is most complete dorsally ; 

 towards the ventral side they grow together, and the junction between the two 

 sets on this side is formed by a thin membranous fold, on which the funnel 

 rests within. 



Between this outer circle and the next envelope, composed of the " labial 

 processes," are two clusters of soft conical papillae, and on each side a group of 

 longitudinal lamellae. 



The labial processes, lying within the outer circle of the oral sheath, are four in 

 number; but the circle they would form is broken, and the two dorsal ones taking 

 their origin farther back, overlap, and thus are partly external to the ventral 

 pair (see fig. 1, a). These labial processes are flattened bands with the terminations 

 at the free ends only slightly marked off from each other, and more in single 

 file than the digital processes. There are a variable number of tentacles in each, 

 usually twelve or thirteen, but sometimes sixteen, and usually more on the inner 

 than on the outer process, but the number is not always equal on the two sides. 

 They have the same structure as the digital tentacles. The band of junction 



