316 INVERTEBRATA OF MASSACHUSETTS. 



nowhere many miles wide ; but this narrow point of land has hith- 

 erto proved a barrier to the migrations of many species of Mol- 

 lusca. Several genera and numerous species, which are separated 

 by the intervention of only a few miles of land, are effectually 

 prevented from intermingling by the Cape, and do not pass from 

 one side to the other. No specimen of Cochlodesma, Montacuta, 

 Cumingia, Corbula, Janthina, Tornatella, Vermetus, Columbella, 

 Cerithium, Py'rula, or Ranella, has as yet been found to the 

 north of Cape Cod ; while Panopsa, Glycy'meris, Terebratula, 

 Cemoria, Trichotropis, Rostellaria, Cancellaria, and probably 

 Cyprina and Cardita, do not seem to have passed to the south of 

 it. Of the 197 marine species, 83 do not pass to the south shore, 

 and 50 are not found on the north shore of the Cape. The re- 

 maining 64 take a wider range, and are found on both sides. 

 Buzzard's Bay and the south shore have as yet been very little 

 explored ; and we may yet expect to find many species peculiar 

 to those localities. 



At least 70 of our species are also found on the transatlantic 

 shores ; and more than 20 of these have been described, by dif- 

 ferent American conchologists, as new species. About 20 may 

 be regarded as intermediate, being found most frequently by fish- 

 ermen about the Banks, Newfoundland, and the islands interven- 

 ing between Greenland and England. 



