740 SEXUAL DIMORPHISM, 



funnel may be faintly detected projecting beyond the anterior lip of the shell. The 

 posterior surface of the hood is concave, in accurate correspondence with the involute 

 convexity {anfractus) of the keel of the shell, and in the concavity there is a semilunar 

 fold which fits upon the blackened involute portion of the shell, towards which it acts as 

 a lubricant. This fold is the nuchal membrane which was compared by Keferstein (1865), 

 and Lankester (1883, Encyc. Brit.) with the cartilaginous nuchal plate of Sepia. I shall 

 have occasion to return to it later. In the specimen which I photographed, the slender 

 tentacles were mostly in a condition of protraction, and when, as in this instance, the 

 animal is stationary although expanded, they are seen to hang down listlessly, but when 

 swimming vigorously backwards are held out straight', like the arms of Octopus under 

 the same circumstances. 



3. Sexual Dimorphism. 



1848. Van DER Hoevex, J. Eenige afwijkingen in den vorm van het hoofd, 

 waargenomen bij een mannelijk voorwerp van Nautilus pompilius. Tijdschr. voor de 

 wis- en natuurkundige Wetenschappen, Deel i. p. 67, PI. 1, figg. 1 — 3. Amsterdam. 



1850. Vax DER Hoevex, J. Contributions to the knowledge of the animal of 

 Nautilus pompilius. Tr. Zool. Soc. London, Vol. iv. part I. pp. 21 — 29, PI. v — viii. 



1856. Van DER Hoeven, J. Bijdragen tot de ontleedkundige kennis aangaande 

 Nautilus pompilius L., vooral met betrekking tot het mannelijke dier. Verhandel. der 

 kon. Akad. Amsterdam, Deel iii., 20 pp., 5 plates. 



1883. Bourxe, A. G. The differences between the males and females of the Pearly 

 Nautilus. Nature, xxviii. p. 580. 



1895. WiLLEY, A. In the home of the Nautilus. Natural Science, Vol. vi., June, 

 1895, pp. 405—414. 



1895. VAYSSiiiRE, A. Sur le dimorphisme sexuel des Nautiles. C. R. Ac. Paris, 

 June, 1895, Tome 120, pp. 1431—1434. 



1901. Deax, B. Notes on living Nautilus. Amer. Natural, xxxv. pp. 819 — 837. 



In Nautilus the sexes are separate, as in all other Cephalopoda, and this dioecism 

 is accompanied by a sexual dimorphism which is determined by the nature of the 

 accessory organs of reproduction and is manifested in the dimensions of the mouth of 

 the shell. 



With regard to the statistical relations of the sexes I found that, judging from the 

 specimens taken in the traps, the number of males exceeds that of the females in the 

 case of N. pompilius, while for N. macromphalus the reverse was true. The excess of 

 females, or hyperpolygjTiy, is a common phenomenon among Cephalopoda, and it is 

 a curious fact that there should be an apparent difference in this respect between these 

 two species of Nautilus. The figures relating to the former species which I have 

 recorded are perhaps more convincing than those which concern the latter. Out of 216 

 specimens of N. pompilius, 150 were male and 66 female. Of 26 N. macromphalus, 



1 This is well shown in some of Dr Dean's figures to which I have already referred. 



