432 NATURAL SCIENCE. June. 1894. 



and unyielding, the obvious result of such pressure is merely to push 

 the animal forward so that it obtains space by lengthening rather 

 than by widening the body-chamber. 



Some caution is, however, to be exercised before accepting the 

 views which Van der Hoeven based upon the evidence of a single 

 specimen. Differences such as he points out do certainly exist 

 between Nautilus shells ; but a careful examination inclines one to 

 regard them as due to difference of age rather than of sex. As N. 

 pompilius grows older, the sides of the shell-aperture begin to swell 

 outwards, so that the width of the aperture is greater towards the 

 inner or dorsal region of the whorl ; and this produces a sinuosity in 

 the margin, which continues to increase with age up to a certain 

 point. May it not then be that Van der Hoeven was deceived by just 

 the same kind of characters in the individual species as we believe 

 Munier-Chalmas and Haug to have been deceived by in the race ? 



To sum up the case in language that has often been objected to 

 in this Journal, and with which its readers cannot, therefore, be 

 wholly unfamiliar, — we suggest that the characters in question are 

 auxologic or bioplastic rather than sexual, being in some cases 

 phylo-gerontic, in others merely ephebic or gerontic, and we conclude 

 that sexual dimorphism has yet to be proved for Ammonite-shells. 



S. S. BUCKMAN. 



F. A. Bather. 



