are freqently bordered by bold rocky headlands separated by 

 crescentic gravelly beaches, the area is remarkably adapted 

 for the differentiation of localized races of Thais. Collections 

 made in Puget Sound, British Columbia and Alaska brought 

 to light many additional variants. 



The publication by Gulick(i905) of his paper dealing with 

 the tree snails {Jchatinellidae) of the Hawaiian Islands greatly 

 stimulated our interest in the studyof our local races of Thais, 

 since it was at once seen that they could be interpreted as the 

 resultant of an evolutionary process in which spatial isolation 

 has played an important role. 



In 1880 there appeared a monographic revision of the Mur- 

 icidae of the world by George W. Tryon Jr. in which the sub- 

 family Purpurinae is thoroughly reviewed. As was customary 

 at that time he placed the forms we now include in Thais in 

 the genus Purpura and in the subgenus Polytropa Swainson. 

 A study of the material at his command led him to the con- 

 clusion that all of the forms related to lapillus, both in the 

 North Atlantic and North Pacific, are variations of a single 

 wide-spread and highly variable species. A quotation from his 

 monograph reads as follows:- 



" The quantity and variety of material before me, embracing 

 a rich series of forms from many localit ies, together with the 

 comparison of numerous descriptions and figures that have 

 been published, induce me to include under this, the oldest 

 name, a very large number of nominal species. As in the case 

 of haemastoma, 1 have considered it preferable to retain some 

 of these names as indicating growth modifications and local- 

 ities; those who take a more conservative view than myself 

 will thus have the names and descriptions at hand to desig- 

 nate these several forms as varieties or species, or even genera, 

 if it so pleases them. I have also endeavored to illustrate a 

 few of the transitional forms". 



A study of abundant material indicates that all of the var- 

 ious forms of Thais found on our Pacific Coast, with the ex- 

 ception of Thais lamellosa, are completely integrated, not only 

 among themsdlves, but also with the innumerable and varied 

 local races of Thais iapillus of the Atlantic. In the case of 



