746 MANTLE ; SHELL ; NUCHAL MEMBRANE. 



has only been taken once. Its shell is occasionally drifted upon the shores of New 

 Britain and elsewhere, but it is much rarer than the others, and is often spoken of as 

 the " king Nautilus 1 ." 



5. Mantle; Shell; Nuchal membrane. 



Under the above heading I shall speak of the formation of the septa which results in 

 the incameration of the shell. The relative importance of this subject may be estimated 

 from the fact that there is an apparently widespread belief which requires correction. 

 It has been suggested, and I think in some quarters adopted as a dogma, that the 

 formation of successive septa is correlated with the recurrence of reproductive periods. 

 This is not the case since, according to my observations, propagation only takes place 

 after the last septum has been formed. Furthermore in spite of the numerous publications 

 which have dealt with the mantle, and the still more numerous treatises on the shell of 

 Nautilus, there is not one paper that I have been able to consult in which the delimitation 

 of the mantle in its relation to various parts of the shell is quite clearly displayed. The 

 relations of the mantle to the animal are now well known and have been so for many 

 years, although in one respect they were erroneously described by Owen (1832), who has 

 the following statement, which must appear strange to all who have handled the animal. 

 He says on p. 9 of his memoir : — " At the ventral aspect of the body the mantle becomes 

 thinner, is prolonged anteriorly, and is perforated by a large aperture through wlrich the 

 funnel passes" [italics mine]. This is one of three principal mistakes" contained in Owen's 

 otherwise wonderful monograph, which was based upon the dissection of a single specimen, 

 and it is perhaps permissible to refer to them, not for the purpose of animadversion but 

 rather for instruction and encouragement. 



The mantle is transparent in the living animal (cf. PI. LXXV.) and consists of two 

 portions, an anterior free pallial fold, which encircles the fore-part of the body like a collar; 

 and a posterior portion, which forms the thin membranous integument of the visceral sac, 

 to which is added an appendix called the siphuncle 3 . 



The integument of the visceral sac has always been reckoned as part of the mantle 

 on account of the fact that the whole of the mantle area is capable of secreting nacreous 

 substance externally, the posterior or visceral portion in particular being concerned with 

 the formation of the septa. The growth of the shell takes place at the free border of the 

 pallial fold, but the whole outer surface of the fold can deposit nacre, as is indicated by 

 the occasional appearance of nacreous intumescences on the inner surface of the shell, and 

 also by the rare occurrence of the phenomenon of true pearl-formation, one example of 

 which came under my observation. 



1 No impression of the contrast between the former and the present distribution of Nautilus as a genus 

 can be better obtained than by an inspection of the superb collection of Jurassic, Cretaceous and Tertiary 

 Nautili which is exhibited at the British Museum (Natural History). Compare also the following publications : — 

 Catalogue of Fossil Cephalopoda in the British Museum, Part I. (1888), by A. H. Foord; Part II. (1891) by 

 A. H. Foord ; Part III. (1897), by A. H. Foord and G. C. Crick. Some seventy species of Nautilus are 

 enumerated in the List of the types and figured specimens of Fossil Cephalopoda in the British Museum (Natural 

 History) by G. C. Crick, 1898. 



2 The other two are the description of non-existent peripheral ganglia and the assertion of a communication 

 between the siphuncle and the pericardium and thence to the exterior through the viscero-pericardial apertures. 



3 See below. 



