742 



SEXUAL DIMORPHISM. 



The sexual dimorphism is more pronounced in Nautilus than in any other recent 

 Cephalopod, with the exception of Argonauta, owing to the fact that the spadix is 



Extreme of Difference. 



Minimum of Difference. 



Fig. 2. Secondary sexual characters in shells of Nautilus pompilius. 

 [Reproduced by favour of Dr F. A. Bather from Natural Science, Vol. vi. 1895, p. 411.] 



•a permanent structure, having a gradual development from the adolescent to the adult 

 stages. It is not periodic in its appearance as is the hectocotylus of the Dibranchs. 

 That its presence should determine a difference in the shape and dimensions of the 

 orifice of the shell, is a fact which has already been made use of by palaeontologists, 

 who now recognise similar sexual variations in the shells of extinct species. Commenting 

 upon my observations and figures (published in 1895) relating to the shells of male and 

 female Nautilus, Professor Howes 1 pointed out that these and the nearly contemporaneous 

 observations of M. Vayssiere bore out the suggestion made by D'Orbigny more than fifty 

 years ago that certain differences in the shells of Ammonites might possibly be indicative 

 of sexual variation. 



1 Howes, G. B. Address of the President, P. Malac. Soc. London, Vol. n., 1896—1897, see pp. 69—71. 



