1916.] 



NATURAL SCIEN< ES OF PHILADELPHIA. 



n:» 



so groups of the principal varieties. The number in each group was 

 tabulated. For future reference the bag of snails was preserved in a 

 tub of formalin. 



A study of the color varieties seems to show that there are a number 

 of colors and patterns that may be combined together in any sort 

 of way. These are as follows: 



(1) Pure white shells that show no trace of color. 



(2) Purple. The color in this ease is diffuse through the shell, 

 but is darker within the aperture. 



Fig. 3. — Map of the North Atlantic Ocean showing the distribution of Thais 

 (Purpura or Xucclla) lapillus. (After Cooke, Proc. Mai. Soc. London, 

 vol. II, p. 193.) The cross-hatching shows the occurrence of T. lapillus. 

 The southern barrier to T. lapillus is no1 clear, but the northern barrier is 

 obviously the presence of ice on the shore 



(3) Chestnut. This color may be pure quite often, but it is- 

 usually combined with purple, forming a chocolate, or with yellow,. 

 forming olive. 



(4) Yellow is a color which, when uncombined with one of the 

 others, makes the shell very conspicuous on the rocks. Owing to 

 its relative rarity and to its being so conspicuous, it often appears 

 in shell collections in proportions that are misleading. Only by 

 collecting all the snails from a given locality can this be avoided. 



(5) Dilute. Every one of the colors mentioned above may appear 

 in a dilute in contrast to its intense form. 



