Occasional Papers of the Museum of Zoology 35 



("Gundlach" Poey) and E. pfeifferiana ("Arango" Pfr.). 

 It differs from all of the species of Pyrgodomus in its narrowly 

 rimate umbilicus and the slight but definite notch on the 

 columellar waU of the peristome. The incomplete, adnate 

 peristome and the Pyrgodomus-like operculum separate Stoas- 

 tomops from Stoastoma, with which genus the radula most 

 closely allies it. I believe it should be placed in the subfamily 

 Helicininae. 



POMATIASIDAE 



Members of this family form the most diversified elements 

 of the moUuscan fauna of the Dutch Leeward Islands. Most 

 of the species are very local in their distribution ; the genus 

 Cistulops is, as far as is known, entirely confined to these 

 islands, while the genus Tudora just reaches the adjacent 

 mainland. 



As large numbers of specimens of most of the species and 

 subspecies were obtained, an attempt was made to study stat- 

 istically their size and shape. For each subspecies, a set of 

 shells, if possible from the type locality, was measured for 

 altitude and minor diameter. The latter was chosen as less 

 variable and more easily measured than the major diameter. 

 As the shells were almost universally decollated, it was also 

 necessary to count the whorls retained; quarter- whorls were 

 taken as unit differences. From these dimensions, the minor 

 diameter index was obtained. As the sexes of all of the 

 specimens had not been noted during removal of the animal, 

 a curve was made by counting the individuals of each minor 

 diameter (in tenths of millimeters). In all cases where the 

 specimens had come from a single locality, this curve was 

 bimodal; and the lowest intemodal point was taken as the 

 dividing line between the two sexes. The obvious error, intro- 

 duced by this approximation, was checked by reference to the 

 modes and by comparison with the curves of species of 

 Tudorata. in which the sexes do not intergrade in size ; I be- 

 lieve it to be practically negligible in most cases, but it in- 

 creases in the smaller species. Using this line of separation, 



