175 



Chickering, jr., found it at Hampton beach, X. H., in 

 1871. The following rude map illustrates the gradual 

 dispersion of this species from Maine southward, with the 

 year of its occurrence marked at the different places. 



EXPLANATION OF MAP. 

 P., PorUamL Maine; Pkov., Provincetown, Mass; W. H., Wood's Holl. 



In 1872, it was observed simultaneously at Salem, 

 Mass., and Provincetown on the end of Cape Cod. This 

 Cape seemed to form a barrier for some time to its passage 

 south, and after rounding the Cape, its progress w^as much 

 slower. Its occurrence at Wood's Holl being observed in 

 1875, and only the past year had it got as far as New 

 Haven. 



A study of the ocean currents north of Cape Cod, 

 which have a southerly course, will account for the rapid 

 dispersal of this mollusk from Maine to Provincetown, 

 Mass. ; while the currents which set easterly south of 



