MANTLE ; SHELL ; NUCHAL MEMBRANE. 749 



The line of insertion of each septum into the wall of the shell is known as the 

 septal suture and follows a broadly sinuous course, well indicated for example in a 

 skiagraph made by Messrs J. Green and J. H. Gardiner and commented upon by Mr B. B. 

 Woodward'. (Cf. PI. LXXVII. fig. 1.) 



In precise correspondence with the septal suture which bounds the fundus of the 

 animal chamber (Wohnkammer) there is an aponeurotic band round the visceral portion of 

 the mantle forming the boundary of the pallial area which secretes the septa (PI. LXXV.). 

 I figured this band in 1896, referring to it as the septal contour", and it has since 

 been further illustrated by Mr L. E. Griffin^ who speaks of it as the " posterior ventral 

 aponeurotic band," having overlooked my previous account^. Recently, however, I have 

 met with a still earlier description of this septal aponeurosis in the work of Appellof 

 (1893), though without illustration. Having referred to the annulus or homy girdle of 

 Owen by which the visceral sac adheres to the shell all round its circumference between 

 and including the shell-muscles, Appellof (up. cit. p. 75) goes on to say : — " Hinter 

 diesen [i.e. the annulus] und sehr deutlich markirt erstreckt sich rings um den Mantel 

 eine zweite Linie, welche der Septalsutur anliegt. Dieselbe tritt durch ihre wei-ssliche 

 Farbe scharf hervor und scheint eine Verdickung im Mantel zu bilden." 



The preceding remarks lead to the consideration of the method of formation of the 

 septa. This is simple enough so far as the septum itself is concerned. What is not 

 so easy to understand is the manner in which the animal glides bodily forwards in its 

 shell so as to leave a space behind it which is destined to become the new air-chamber. 



The foundation of a new septum consists of a very thin, easily torn membrane, 

 presumably conchyolin, which appears as a cuticle over the entire septal area of the 

 mantle, i.e. the area limited by the septal aponeurosis. Upon this membrane nacreous 

 matter is deposited commencing from the septal suture, and the membrane itself remains 

 as a thin pellicle over the hinder surface of the septum. This pellicle appears to better 

 advantage in some specimens than in others. It is a striking circumstance, upon removing 

 a young Nautilus from its shell, to find that the septum which constitutes the fundus 

 of the animal chamber is composed of soft membrane only without any trace of nacreous 

 substance. When a calcified septum is substituted for the primary membranous septum 

 by the deposition of nacre, it is at first exceedingly thin and ft-agile, becoming gi-adually 

 thickened by further secretion, but the size of the air-chamber is determined from the 

 commencement of calcification and undergoes no subsequent change. 



If the homy membrane which covers over the muscular impressions in the shell 

 be removed, it will be found that each muscle-scar consists of a series of close-set 

 concentric lines exactly like the gi-owth-lines of the shell, not however continuous with 

 the latter but quite independent, superposed upon the growth-lines of the shell, and 

 traversing them at an angle. These superadded lines may be called the growth-lines 



1 P. Malac. Soc. London, ii., 1890, p. 179, PI. xv. 



•^ Willey, A., " Letters from New Guinea." Quart. ./. Micr. Sc, Vol. 39, August 189G, p. 170, fi^. Ifi (see 

 also below) . 



" Griffin, L. E., "Notes on the Anatomy of Nautilus pompilius." Zool. Bull. (Boston), Vol. i., 1897, 

 p. 148, fig. 1. 



^ It is indicated without a description in a drawing by .1. van dur Hoeven, Tr. Zool. Soc, iv.. Part I., 1850. 



98—2 



