748 MANTLE ; SHELL ] NUCHAL MEMBRANE. 



There are several features in the shell which indicate that it has reached its limit 

 of growth and that its inhabitant is therefore mature. The lip of the shell which has 

 hitherto been thin and fragile gradually becomes thickened, and a black border is 

 ultimately formed round it on the inside in continuity, at the umbilicus, with the 

 coating of black varnish which covers the involute convexity or anfractus of the shell 

 from the earliest stages. The septa, as seen in sections through the shell, become 

 progressively thicker from the earliest to the latest, so that the last septum which 

 precedes sexual maturity is, when completed, the thickest of all 1 . Another striking 

 character which indicates the termination of the chamber-building activity of Nautilus 

 is found in the fact that the last air-chamber of the finished shell is generally smaller 

 than the one which preceded it, due no doubt to advancing age, but one of my bisected 

 shells of N. macromphalus provides an apparent exception to this rule in that, though 

 the black edge has been added to the lip of the shell, the final chamber is not smaller 

 than the penultimate. It was formerly supposed that the most recent chamber was always, 

 at its first formation, smaller than its jDredecessor, even in young growing shells, and 

 this assumption was used as an argument in support of a theory of shell-growth by 

 intussusception [Riefstahl], but it is not so, and the error has been duly corrected 2 . 



The total number of chambers constructed seems to vary considerabl)-. Three shells 

 of N. pompilius have 36, 34, and 33 chambers respectively, two of N. macromphalus have 

 28 and 27, one of N. umbilicatus 32. 



Although the umbilicus of N. pompilius is closed externally by a deposit of callus 

 it is well known that a section through the shell reveals the presence of an umbilical 

 fossa in consequence of which the initial chamber of the shell has a free outer surface. 

 In the middle of this surface when isolated by chipping away the rest of the shell 

 there is a sub-rotund area with a slightly raised oval boss in the centre of it, and in 

 the middle of the boss a shallow elongate depression, the whole somewhat resembling 

 the structures known in plants as bordered pits. This is called after its dis- 

 coverer as Hyatt's scar 5 and is the principal feature of the shell, concerning the nature 

 of which and its relation to a possible protoconch we might expect to acquire special 

 information from a study of the embryonic stages of development 4 . 



At the median dorsal border of the earlier septa, from the third to the twenty- 

 second or thereabouts, there is a deep pit which occasions a prominent lobe to project 

 backwards into the preceding chamber from each septum. This dorsal lobe of the 

 septum was observed by Valenciennes and later writers, and an excellent illustration has 

 been furnished more recently by Dr Appellof 5 . 



1 This condition is not shown in many published figures of the shell, but is clearly reproduced in the 

 collotype published by Dr B. von Lendenfeld in his " Bemerkung zu Biefstahl's Wachsthumstheorie der 

 Cephalopoden-Schalen," Zool. Jahrb. Syst. in., 1888, pp. 317, 318, Taf. ix. 



2 Bather, F. A., "The Growth of Cephalopod Shells." Geo!. Mag., iv., pp. 446—449, 3 figg., 1887; also 

 by the same author, "Professor Blake and Shell-growth in Cephalopoda." Ann. Nat. Hist., June 1888, 

 pp. 421—427 ; also same Journal for April, 1888, pp. 298—310. B. von Lendenfeld, op. cit., 1888. 



3 Hyatt, A., "Fossil Cephalopods of the Museum of Comparative Zoology: Embryology." Bull. Mus. 

 Harvard, in., pp. 59—111, PI. in. fig. 1, 1883. 



4 Cf. Bather, F. A., "Cephalopod Beginnings." Nat. Sci., v., 1894, pp. 422—436. 



5 Appellof, A., "Die Schalen von Sepia, Spirula und Nautilus." Svenska Ak. Handl. xxv., Stockholm, 1893; 

 see Taf. x. fig. 2. 



