BY H. L. KESTEVKN. 71 



was intended to embody is as follows : — In as far as the size 

 of the protoconch is a measure of the length of the pelagic 

 existence of the species forming it, so also will it be, in most 

 cases, a measure of the distribution of the species. This mode 

 of expression of the idea becomes necessary with the dis- 

 coveries that a protoconch may be largely ananeanic, post- 

 larval, and that in some cases, e.g., Triforis, several of the 

 early whorls of the shell, differing only in size from all which 

 succeed them, may have ben formed by the larva. 



The Heterostrophe Apex. 



The Heterostrophe Apex is of particular interest, because 

 it apparently stands out as an apex which is perfectly trust- 

 worthy for taxonomic purposes ; examination of the facts 

 shows, however, that the heterostrophic apex is confined 

 neither to a single genus nor indeed to a single family ; and, 

 moreover, that it is not constantly present in the species of 

 the several genera in which it has been described as a diagnos- 

 tic character, e.g., Eulima. It was on this evidence that Tate 

 and May assigned to the Pyramidellidae their new genus 

 Pseudorissouia of which they say: "Briefly this new genus is 

 a Pyramillid with a Rissoina aperture or a Rissoinid with a 

 Pyramilloid nucleus (apex)." They also say that it has the 

 "spire and aperture of a Rissoina, and are induced by" the 

 higher value of the nuclear character (over adult characters) 

 in classification to place it, in Pyramidellidse, near Euli- 

 inella." I would not be understood to say that this classifi- 

 cation is wrong : that question must remain until the mollusc 

 itself is known, but I would point out that it is based on a 

 wrong premise. 



The heterostrophic character has arisen out of a twisting of 

 the mollusc in the shell. This may be illustrated by a refer- 

 ence to EiselJa and Littorina. In the former genus, the 

 dorsum of the body is related to the lower aspect of the penul- 

 timate whorl, and the retractor muscle attached to the floor of 

 the ultimate whorl. In Littorina, the dorsum of the body is 



