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 twenty 

 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 migi'ation 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 Panopsea, Glycymeris, Cyprina, 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, Ra- 

 nella, and Pyrula, do not pass to the north of it. Of the two hundred 

 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 a fact 

 claiming particularly the attention of geologists, in estimating the 

 value of shells as a 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, and tame their 

 exuberant fancy to the sober views of inductive philosophy. 



The future student of American molluscs will owe an unending obli- 

 gation to Dr. Gould, for the labor and science he has brought to the 

 completion of this task. He has accumulated within the compass of 

 a convenient volume, all that was valuable in the published memoirs 

 of his predecessors, scattered through the pages of scientific journals 



* Boston Journal of Natural History, Vol. Ill, No. 4. 



