lO 



Thais lamellosa the writer is of the opinion it is specifically 

 distinct. Although populations of this species are commonly 

 found mingled with those of various forms of the lapillus type 

 no clear-cut evidence of cross-breeding has been observed. In 

 cases where the sculptural pattern of the shell in T. lamellosa 

 approximates that frequently found in T. lapilluSj this may 

 readily be explained on the basis of their common Purpurine 

 ancestry. The distinctness of the two groups is also borne out 

 by the palaentological evidence, since both types occur in 

 rocks of Tertiary origin. 



In view of the above considerations the writer decided to 

 deal with the problem of Thais lamellosa at this time, and 

 leave the more complicated case of our west coast forms of 

 Thais lapillus for future treatment. 



In 1910 Miss Bertha Challis, at that time a graduate stu- 

 dent in the Department of Zoology, became associated with 

 the writer in the study of the Thais material, and eventually 

 decided to utilize some of our findings in a thesis for the Mas- 

 ter of Science degree. In this connection Miss Challis made 

 a number of excellent photographs of groups of the shells of 

 Thaisy some of which have been utilized in the present report. 

 The Challis report was not published, but included our find- 

 ings at that time which were, as has been indicated, markedly 

 influenced by the papers of Tryon (1880) and Gulick(i9o8). 

 During the intervening years the writer has continued to ac- 

 cumulate representative Thais material from all possible areas 

 of the Pacific Coast, as well as material from the Atlantic for 

 comparative purposes. The writer has also had an opportunity 

 to examine the extensive collections of Thais in the United 

 States National Museum which formed the basis for the study 

 of thisgroupbyDr. W. H. Dall, and those in the Philadelphia 

 Academy of Natural Sciences administered by Dr. H . A. 

 Pilsbry.The molluscan collections in the California Academy 

 of Sciences in San Francisco, and in the Geological Museum 

 of Stanford University also include extensive series of Thais 

 material from west coast areas. 



