REPORT OF THE DIRECTOR I919 



69 



growth, depth of septa and camerae; and hkewise does the third 

 form with wide open aperture fail to exhibit any other differences 

 suggestive of specific separation. This third form is also of smaller 

 size. 



Fig. 5 Fig. 6 



Figs. 5-6a Oncoceras pupaeforme 

 fig. 6 Mature male; fig. 6a Immature female. 



Fig. 6a 

 Fig. 5 Mature female; 



We consider these three forms as belonging to a single species (to 

 be described in a later publication as Oncoceras pupae- 

 forme n. sp.) ; the larger and smaller forms with contracted 

 apertures representing the mature females and males, and the smaller 

 forms with open aperture the immature females. 



The conclusion that the larger shells contain the females of the 

 species and the smaller ones the males, is based on the observation 

 that in the cephalopods (E. Ray Lankester, V, p. 319), " as a rule the 

 males are more slender (e. g. Loligo media) or smaller than 

 the females." In Argonauta, where the maximum of sexual dimorph- 

 ism is found, the females are as much as 15 times as long as the 

 males. 



In Nautilus, which would be the nearest relative among recent 

 cephalopods to use for comparison, the difference between the conchs 

 of the two sexes seems to be still subject to some doubt. Bashford 

 Dean (1901, p. 819) describes the shells of the females as frequently 

 a little more arched and ventricose than those of the males, while 

 Willey, on the contrary, considers the narrower and higher shells as 

 the female ones. Other authors (see Ray Lankester, loc. cit.) 



