﻿8 BRITISH FOSSIL CEPHALOPODA. 



between the external pair, which are free on the ventral side and united dorsally, 

 is formed by two organs with a very large number of longitudinal folds, looked 

 upon by some as rudimentary tentacles, but otherwise of unknown office. The 

 internal pair are in like manner joined on the ventral side by a longitudinally 

 folded band (fig. 1, a), which is largely supplied with nerves, and was considered 

 by Owen to be an olfactory organ before the true seat of that sense was discovered by 

 Yalenciennes. The bands on this side are much fewer than on the other, but they 

 are otherwise alike. 



In the male the oral sheath presents some remarkable modifications from this 

 structure. The outer circle has nearly the same characters, except that the four 

 dorsal digital processes are more markedly separated from the rest, and lie outside 

 and a little farther back, and there is no cluster of papillae between the digital and 

 the labial processes. The dorsal pair of the latter are divided on each side into two 

 parts : the more remote and least dorsal have each eight tentacles, of which two are 

 smaller than the other six, but the more adjacent parts are placed outside of the 

 others ; that on the right side has four tentacles of usual character, with three of 

 the sheaths more united to each other than to the fourth, but on the left side the 

 corresponding four tentacles are modified to form a peculiar body known as 

 the " spadix." This is more separate than the other divisions from the rest of the 

 labial process, so as really to stand between it and the digital processes. The spadix 

 is a great conoidal body about two inches long ; on the outer side towards the free 

 end is a broad oval patch with a raised border, within which are a number of round 

 apertures leading into as many small follicles set perpendicularly to the surface. A 

 section of the spadix shows it to be composed of four tentacles, whose structure is the 

 same, on a larger scale, as that of the rest ; the innermost one of these is only 

 united to the others by a membrane at the base ; the other three are coalesced into 

 a single mass. The dorsal junction of these labial processes shows no longitudinal 

 folds, it is only a membrane with a reticulate surface. The ventral pair of labial 

 processes are wanting, as such, in the male ; but their place is taken by another 

 peculiar organ. On the ventral side of the buccal mass, and in close apposition 

 with it, is a deep fold of membrane, with a slit, below, in a transverse direction : 

 within this fold is a compound organ consisting of two nearly flat halves, each bean- 

 shaped, and with their convexities turned towards each other. The convex 

 margins are divided into a variable number (7 to 11) of tetragonal imperforate 

 processes, and in addition fourteen other very thin lamina? running obliquely 

 towards the line of junction. The office of this organ is unknown, but in form it 

 is like a pair of retracted and rudimentary labial processes. 



Within this complex oral sheath, in either sex, lies the buccal mass. This is, 

 in the first place, covered by a fold of membrane rising from the re-entering angle 

 of the former, and developed at its free margin into pointed and jagged processes 



