744 SPECIES AND RANGE. 



• Sowerby, with narrow perforate umbilicus, N. Tnacromphalus Sowerby, with rather wide 

 subperforate ' umbilicus, N. umhilicatns Lister, with very wide perforate umbilicus. 



Judging from the known facts relative to the coiling of the shells of extinct 

 Ammonoids and Nautiloids and also from the fact that the young shells of N. pompilius 

 are perforate, the umbilical region becoming subsequently covered over by callus, I suppose 

 it is natural to conclude that the form of the shell in N. umbilicatus is the most primitive 

 among recent species, while that of j\^. pompilius is the most highly adapted. 



If inquiry be made into the nature of the adaptation which has resulted in the 

 complete closure of the umbilicus in the latter species, it will be found as a fact that 

 an exposed deeply excavated umbilical region, such as is presented by the shells of 

 JV. macromphalus and N. umbilicatus, is liable to be selected as a nidus for the attachment 

 of all kinds of sedentary animals, Serpulids, Cirripedes, Ostreids, Bryozoa, Foraminifera, 

 and others. 



The fouling of the shell resulting from the incrustations of these foreign bodies will 

 not only serve to initate the sensitive edge of the mantle in theii- vicinity, but ^yi\[ 

 also materially hamper the swdmming of the Nautilus. (PI. LXXIX. fig. 1.) 



The deposition of callus over the umbilicus in ^V. pompilius effectually repels the 

 visits of unbidden guests. In ^^e^v of these considerations it is interesting to note that 

 it is precisely in the umbilical region that the shell of iV. pompilius exhibits marked 

 variations, to which I have called attention (1896), and I came to the conclusion that 

 iV. stenomphalus was simply an extreme variety of this species. Shells which are assigned 

 to M. stenomphalus also vary in the diameter of the umbilicus, and I named a form 

 of which I obtained one specimen of the shell only, through the kindness of the 

 Hon. M. H. Moreton, at Samarai, N. pompilius, var. Moretoni, but I have since been shown 

 a specimen in the Manchester Museum by Mr W. E. Hoyle which is identical with mine. 



In 18.55 Macdonald described the anatomy of a species from New Caledonia, which 

 he erroneously called N. umbilicatus, subsequently recognising his mistake, and correcting 

 the name to that of X. macromphalus^. Macdonald's investigation of the anatomy of 

 N. macromphalus resulted, as is well kno\\Ti, in the discover}- of the otocysts, but revealed 

 no appreciable specific differences between it and N. pomjiilius so far as the soft parts 

 are concerned. In fact the animals of these two species are really indistinguishable, and 

 I was therefore very pleased to come into the possession of a single mutilated specimen 

 of X. umbilicatus accompanied by its shell, which had been picked up from the surface 

 of the sea, not far from Milne Bay in British New Guinea, and to find that this species 

 differed noticeably from its congeners by the character of the hood, the gibbosities of 

 which have the form of flat-topped angular areas separated by deep grooves, producing 

 a pronounced tessellated appearance. (PI. LXXVIII. fig. 3.) 



In comparison -with its former world-wide distribution, evidenced for example by the 

 abundance of its remains in the London Clay, the present restricted range of the genus 



1 If the shell is coiled in such a manner that a space is left between the first chamber and the first 

 turn of the spiral so that light may be seen through the pin-hole aperture, the umbilicus is said to be 

 perforated. In the shell of ^V. macromphalus there is a pit on each side but it does not extend through 

 from one side to the other. 



- Macdonald, J. D., "Further observations on the anatomy and physiology of Nautilus." P. R. Soc. 

 London, viii., 1857, pp. 380—382. 



