36 University of Michigan 



the shells were arranged in columns, according to sex and 

 number of whorls, and the means for altitude, minor diameter 

 and index were computed for each class, and for the total 

 population of each sex. 



In the tables of dimensions (tables IV-VII) are presented 

 the data for the entire population and for some one of the 

 whorl classes. As might be expected, the mean altitude in- 

 creases and the mean minor diameter index decreases with 

 the number of whorls retained. The males retain a smaller 

 number of whorls than do the females; this appears to be a 

 matter of sex and not of comparative size, as the mean dia- 

 meter is usually quite constant in those whorl-classes near the 

 mean, although very small shells often retain a smaller number 

 of whorls than do the veiy large ones of the same sex. In 

 Cistulops raveni aruhana, complete indiyiduals of both sexes 

 were obtained, and the females appeared to develop a greater 

 number of whorls than did the males. In the larger species 

 studied, the males are slightly more slender than the females 

 with the same number of retained whorls ; this also may mean 

 that the males tend to develop a lesser number of whorls. 

 Complete males were obtained of Cistulops raveni raveni, 

 Tudora maculata, Tudora fossor fossor, and Tudora fossor 

 canashitensis ; the data for these are given. In the other sub- 

 species, the number of whorls is that estimated by the com- 

 parison of young shells and modal, decollated females ; I sus- 

 pect that the male would average i/^ to 1 whorl less. Th'3 

 rather large variation in all dimensions is probably due, in 

 large part, to variation in the number of whorls, which appears 

 to be rather characteristic of these desert sheUs (compare 

 Cenon uva). The apparent variation is also partially 

 caused by the rather large whorl-classes chosen; data from a 

 single whorl-class thus give a markedly flat-topped curv^e. For 

 these reasons, apparent intergradation, in the indices of two 

 species, does not necessarily indicate actual intergradation in 

 shell form. 



