MANTLE ; SHELL ; NUCHAL MEMBRANE. 751 



specimens a wide interval between them, and in the centre of this intervening pallial 

 tract may be seen the line of insertion of what Huxley' named the pallio-visceral ligament, 

 that is, the membrane with the three fontanelles which incompletely divides the pericardial 

 from the visceral portion of the coelom. In young examples the interval between the 

 septal and annular bands is both relatively and absolutely much less considerable. These 

 relations also leave their impress on the shell. 



The occurrence of muscle-lines on the shell- wall behind the posterior border of the 

 actual muscle-scar, to which I have alluded above, seems to me to be specially worthy of 

 note as indicating gradual normal growth. So true is this that if the horny membrane 

 is lost, especially in young shells, it is sometimes not easy to discern the posterior limits 

 of the muscular impressions, and Appellof (1893) states that in several small dried shells 

 of N. i^ompilius he has sought in vain for an indication of the muscle-scar and annulus. 



The above description of the growth of Nautilus is in substantial accord with the 

 observations of Keferstein (1865), except with regard to his and Appellof's hj^aothesis 

 of a local resorption of muscular tissue. So far as I can ascertain, my record of the fact 

 that the hard nacreous calcified septum is actually preceded by a soft membranous 

 septum is new. 



The three facts which are necessary to remember when attempting to form a subjective 

 conception of the mode of growth of Nautilus are as follows: — 



(1) Growth in bulk of the animal taking place at regular intervals, terminated 

 respectively by the formation of a new septum. 



(2) Rigidity of the shell necessitating a forward movement of the too bulky animal. 



(3) Faculty of secreting gas. 



A few words remain to be said concerning the anterior free pallial fold and the 

 nuchal membrane. The free border of the mantle forms a continuous collar round the 

 entire periphery of the animal, and may be considered in three divisions, ventro-lateral, 

 umbilical, and dorsal. The contour of the mantle-edge accurately coiTesponds with that 

 of the mouth of the shell, and the mantle is never reflected upon the external surface of 

 the shell, except in the region of the involute convexity (anfractus). By a coincidence 

 it happens that in the recent species of Nautilus the contour of the lip of the shell is 

 a close repetition, on an enlarged scale and in a different plane, of the shape of the 

 septal suture. 



The dorsal fold of the mantle was described and named by Owen but not correctly 

 figured by him, a better illustration being furnished by Van der Hoeven in 1850 in 

 the Transactions of the Zoological Society. It is reflected over the involute convexity 

 of the shell and apparently secretes the layer of black varnish which distinguishes this 

 part, so that the sharp line with which this layer terminates denotes the extent to 

 which the mantle may be reflected over this portion of the shell. I received the 

 impression that the dorsal fold of the mantle does not always cover the black area but 

 may be retracted so that the nuchal membrane which adheres to the hinder concavity 

 of the hood comes to play upon this smooth surface. Frequently the animal executes 



1 Huxley, T. H., " On some points in the anatomy of Nautilun pompilius." J, Liitn. Soc, iii. 1859, 

 pp. 3G — 44. 



