4S 



and the individual snails mature to a much greater size. This 

 condition is illustrated in Plates lvii and lviii, where speci- 

 mens are shown derived from submerged reefs that are never 

 uncovered by the falling tide. It has also been observed that 

 Thais inhabiting the piling of docks, to which great masses of 

 barnacles and mussels are commonly attached, seem to find 

 this situation very favorable for rapid growth, as may be seen 

 in Plates xxxiii and xlviii, typical of such situations. 



However, even in those populations in which the relation to 

 environmental conditions seems rather obvious, there is much 

 variation among the individual snails, and in all populations 

 the same is true. In some populations we find a remarkable 

 gradation in size as shown in Plate xxi, where it will be seen 

 that even the smallest specimens are fully mature, since the lip 

 of the shell is fully formed. Two possible explanations suggest 

 themselves, one that we have a common environmental condi- 

 tion acting upon a variable hereditary factor for size, the other 

 that variations exist in the ecological conditions within the 

 localized area inhabited by a group of snails, which affect the 

 nutritive opportunities of the individual animals. It is readily 

 seen that such ecological niches exist in the intertidal zone, 

 and it is a question as to whether this condition is counter- 

 balanced by the migratory powers of the snails in escaping 

 from an unfavoable environment. At the breeding season the 

 animals traverse considerable distances to assemble in large 

 numbers for the deposition of their eggs. The behavior of the 

 snails, both in their natural habitat and when confined in an 

 aquarium, suggest considerable powers of adjustment. Even 

 in the populations inhabiting submerged reefs the same type 

 of size variation occurs. 



The populations o( lamellosa distributed along an extensive 

 coastline from California to Bering Sea, exhibit the phenome- 

 non frequently seen in polymorphic species in that populations 

 in closely adjacent areas may be very unalike, while others 

 although widely separated may be very similar in appearance. 

 This would indicate that the controlling genetic elements are 

 limited, and by chance combinations may produce similar 

 phenotypes. 



