1913.] NATURAL SCIENCES OF PHILADELPHIA. 17 



range from five to five and three-tenths whorls, and in rare cases 

 one may reach five and one-half whorls. As in the case of L. granu- 

 losa, the reduction in size is accompanied by a reduction in the number 

 of whorls, and the animal matures at an earlier stage of shell develop- 

 ment than is the case in the larger normal forms. The small speci- 

 men from Santa Cruz Road which approximates in size the Montego 

 Bay specimens had only five and three-tenths whorls, as they have, 

 and a few other Santa Cruz Road specimens, which are small, run 

 about five and seven-tenths whorls. 



The Causes of the Variation in Size and Sculpture. 



In the case of the variation of the two species herein described 

 it is plain that we have to deal with two different kinds of variation: 

 (1) A reduction in size from the normal type and (2) a change of 

 sculpture. The cause of the change of size is the same in both 

 cases and has already been stated. The reduction in size is undoubt- 

 edly brought about by the drier and more arid environment which 

 retards the growth of the individual; superimposed upon which is 

 the regular periodicity of climatic changes due to the changing 

 seasons, which induces the development of the reproductive organs 

 at a given time in the year, without much regard to the size the in- 

 dividual has attained. Thus the forms living under arid conditions 

 will have onl,y reached the beginning of the sixth whorl (or passed 

 the completion of the fifth whorl) when the enforced aestivation 

 brought on by the dry season commences. During the dry season 

 all of these snails are more or less inactive, and during this resting 

 stage the genitalia are developed. The lip probably commences 

 to develop about the same time. Forms living under optimum 

 conditions have reached the beginning of the seventh whorl when this 

 occurs; they will have an extra whorl as compared with the forms 

 living under arid conditions, which would have had many less 

 feeding and growing days, and which latter, no doubt, have passed 

 through many short periods of aestivation during their period of 

 growth, which was frequently interrupted by the dry spells between 

 showers. Probably from the repeated aestivation periods that these 

 dwarfed forms must pass through, there has been produced an 

 actual decrease in the size of the embryo, as is indicated by the re- 

 duction in the size of the protoconch; and this may mark the fixing 

 of the small race, even though their environment may change; 

 but this dwarfing of the embryo is not needed to explain the reduction 

 in size. 

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