380 Bibliography. 
species. Peculiar pains were taken to give accuracy to this most dif- 
ficult branch, and a correspondence and exchanges were opened with 
all the principal conchologists in this country, and several in Europe, 
for the purpose of ascertaining what species were peculiar to the ter- 
ritory of Massachusetts, and what were common to both shores of the 
Atlantic. By these means it has been proved, that at least 
species of shells which had been described as new by our naturalists, 
were well known before in Europe, while seventy species, according 
to the present state of our knowledge, are peculiarly American. And 
what is a little singular, just about the same number are believed to be 
common to both countries. That narrow tongue of land called Cape 
Cod, which forms a long curved beach, extending for forty or fifty miles 
to the south and east of Boston, and is scarcely twenty miles over in the 
widest part,—usually much less, presents a great natural boundary be- 
tween two seas, and by its peculiar form and extent gives fine opportu- 
nities for observing the limits of migration for many species of marine 
shells. ‘* Many whole genera do not pass from one side to the other 
of this limit. Thus no species of Panopwa, Glycymeris, Cyprinay Ter- 
ebratula, Cemoria, Cancellaria, Rostellaria, or Trichotropis, has yet 
been found to the south of the extreme point of Cape Cod ; while Cor- 
bula, Conchlodesma, Cumingia, Montacuta, Tornatella, Cerithium, ol 
nella, and Pyrula, do not pass to the north of it. Of the two! 
and three marine species, eighty one do not pass to the south, and thirty 
have not been found to the north of the cape, though many of them 
approach within a very few miles of each other. The remaining ninety 
two species take a wider range, and are found on both sides.”* — 
The entire extinction of certain species and even genera at particular 
localities where they have abounded within the human era, is @ fact 
claiming particularly the attention of geologists, in estimating the 
value of shells asa means of determining the age and extent of geo 
logical formations. It will be remembered by geological readers, that 
some facts of this character in Scotland, early excited the notice of the 
distinguished author of the “ Principles of: Geology,” whose calm and 
philosophic views of the phenomena of nature have done so much to 
recall geologists from the fairy land of wild speculation, me ame the 
serial eed tothe: sober: ‘views of inductive erred 
im, sea snidingr ob 
