32 



Geologic and hydrographic factors. 



For the long period of time during the Pleistocene from 

 which we have a record of the existence of the ancestral forms 

 of Thais lamellosa^ there have been momentous changes in the 

 level of the sea and the climatology of the area, which must 

 have profoundly influenced the distribution of the species. 



THE FOOD HABITS OF Thais lamellosa. 



All who have studied the ecology of Thais appear to be in 

 agreement that the basic food of the animals is barnacles. Dr. 

 Moore has proved that T. lapillus can live indefinitely upon a 

 diet of MytiluSy and presents evidence that this is an impor- 

 tant factor in determining the coloration of the snails in their 

 intertidal habitat. Moore has also expressed the view that the 

 newly hatched snails occupy a lower zonal distribution than 

 the parent forms, utilizing the small annelid SpirorbiSy with 

 other minute attached invertebrates. 



Some authors have reported lapillus as a predator on oys- 

 ters, but this belief does not appear to be well founded, and 

 some of the other reports regarding the food habits of lapillus 

 seem to have been handed down without critical examination 

 and should be rechecked. 



As regards the food habits of lamellosa, no critical studies 

 appear to have been made. In all cases where individual snails 

 were checked they were feeding on barnacles, even though 

 clusters of Mytilus were close at hand, which indicates that 

 barnacles constitute their preferential diet. 



In some instances extensive reefs constituted almost exclu- 

 sively of MytiluSy existed apparently without attracting a 

 population of lamellosa. 



In some areas in the San Juan Islands there are extensive 

 submerged beds of the mytilid Modiolus modiolus L., but no 

 Thais appear to be present. 



Contrary to the reports as to the method by which lapillus 

 attacks barnacles, the writer found lamellosa utilizing its rad- 

 ula to perforate the protective plates of the barnacles and the 

 shells of mussels. 



