i894. SEXES IN AMMONITES. 431 



the situation of the animal with regard to the shell was much the 

 same in the Ammonites, and that any effect produced in the Ammonite 

 shell by sexual difference would probably also be found in that of 

 Nautilus. 



Owing to the somewhat curious fact, which may of course be 

 mere coincidence, that the large majority of specimens of Nautilus 

 examined by eminent naturalists have been of the female sex, not 

 much has been said upon this subject by zoologists. J. D. Mac- 

 donald, however, in his account of the anatomy of Nautilus iimbilicatus 

 [Phil. Trans., 1855, p. 277), has a sentence which, though its logic is 

 obscure, certainly bears on this question. He writes : "The speci- 

 men of N. umhilicatus examined proved to be a female, a fact which 

 may serve to modify the views of those who, adopting the ingenious 

 speculations of d'Orbigny with reference to the sexes of the Ammonites 

 as indicated by the characters of their shells, apply them also to the 

 several kinds of Nautili known." He appears to be alluding to the 

 belief that the species known as N. nmbilicatus was nothing more than 

 the male of N . pompilius. Still the fact that it was really a true 

 species, though it might upset that particular belief, would not 

 conflict with the idea that there might be some differences between 

 the shells of the two sexes. 



An attempt to demonstrate some such difference was made in 

 the same year by J. Van der Hoeven {Verh. d. kon. Nederl. Akad. van 

 Wetensch.), who had at last obtained a male Nautilus pompilius. After 

 describing various external sexual characters of the animal, he 

 proceeds : — " It seems to me moreover as not improbable that in 

 the shape of the hood there is a sexual distinction, and that this, 

 when of the same average length, is about two centimetres shorter in 

 the male individual. With this is also connected a distinction in the 

 shape of the shell ; in the male individual this is broader and more 

 globose at the aperture, more laterally compressed in the female 

 animal. Also the edge of the aperture of the shell in the male 

 animal is, as it seems to me, more strongly sinuous; in the female 

 animal more regular." It will be noted that these points of difference, 

 assuming them to be correct, by no means agree with the suppositions 

 of de Blainville, d'Orbigny and others with regard to Ammonites ; 

 and in such a case surely no one will venture to exclaim " So much 

 the worse for the Nautilus I " Moreover, if one may judge from the 

 drawings by A. G. Bourne which adorn Professor E. Ray Lankester's 

 article on Mollusca in the "Encyclopaedia Britannica," both the male 

 animal and its shell are proportionately broader than the female. It 

 is indeed by no means necessary to suppose, as did the older writers, 

 that the swelling of the female organs involves a widening of the 

 shell. The shell of the body-chamber must necessarily have been 

 formed before the development of the ova, and any pressure caused 

 by the enlargement of the soft parts must act on the soft parts them- 

 selves quite as much as on the shell. Since, however, the shell is hard 



