752 MANTLE ; SHELL ; NUCHAL MEMBRANE. 



violent up-and-down movements in the shell, and at such times my observation led me 

 to believe that it was the nuchal membrane which plied against the black surface, and 

 I even thought that this membrane was responsible for the black deposit 1 . But the 

 evidence of sections which I have since made is against this view, and I now think 

 that the older writers who inferred that the black pigment was secreted by the dorsal fold 

 of the mantle, and Professor Joubin 2 who described the gland-cells in section, are right. 

 The epidermal cells on the outer side of the dorsal fold of the mantle are obviously 

 glandular and more columnar than those on the inner surface, whereas the epidermis of 

 the nuchal membrane is of the nature of a mucous membrane and is lower on the 

 side which is next to the shell than on the obverse side. I have often seen the 

 mantle being withdrawn from the upper portion of the convexity of the shell when 

 I have been handling living Nautili, but I am unable to state the precise conditions 

 under which the dorsal fold of the mantle and the nuchal membrane respectively come 

 into contact with the black area. There are moreover distinct indications in the 

 structure of the shell itself showing that the mantle is not always under normal con- 

 ditions kept stretched over the entire black area, since there is another margo limitans 

 situated at about one-third of the distance between the dorsal border of the septal 

 suture and the margo limitans of the black area. The second line to which I refer 

 represents the limit of a thin nacreous deposit which is laid over the black deposit on 

 that part of the involute convexity or anfractus (Van der Hoeven) of the shell which 

 faces downwards when the shell is held with the mouth directed upwards. (PI. LXXVII. 

 fig. 1.) 



The facts may be summarised by saying that the free lip of the shell constitutes 

 the margo limitans of the ventro-lateral fold of the mantle, while the anfractus presents 

 two margines limitantes of the dorsal pallial fold, namely, the limit of the nacreous 

 deposit and the limit of the black varnish. In quite small shells 33 mm. in diameter, 

 measured in section from the anterior lip to the convex posterior surface (the line of 

 maximum diameter passing some distance below the umbilicus), the nacreous deposit 

 extends halfway over the black area; in older and in adult shells it occupies no more 

 than one-third of the black area. 



The dorsal fold of the mantle has therefore two functions to perform, the deposition 

 of black varnish over the whole of the anfractus (involute convexity) and of nacre upon 

 the lower portion only. 



With regard to the topographical relations of the nuchal membrane I can only 

 call to mind one complete and accurate account of it, namely, that given by Professor 

 Lankester in the article "Mollusca" (Encyc. Brit, 9th edit., 1883). Owen (1832, p. 12) 

 correctly describes its position and shape as a " semilunar ridge," but adds that " it is 

 from this ridge that the mantle is immediately continued to form the concave [dorsal] 

 fold." Van der Hoeven (1850) while adding a new fact repeats Owen's mistake as to 

 a direct connection between the nuchal membrane and the dorsal fold of the mantle in 

 these words : — " Under that fold [the dorsal fold] is a smaller plate of nearly the same 



1 Willey, A., "Letters on Nautilus, 1896." Quart. J. Micr. Sc, Vol. 39, p. 172. 



2 Joubin, L., " Recherches sur la coloration du tegument chez les Cephalopodes, 4 me partie, Glande 

 secretant le vernis noir chez le Nautile." Arch. zool. exper. (2) x., 1892, pp. 319 — 324. 



