MOLLUSCA. 315 



RECAPITULATION. We find that there are, in all, 268 species, 

 of which there are of CIRRI'PEDES 12, CONCHI'FERA (bivalve) 

 97, BRACHIO'PODA 2, GASTERO'PODA (mostly univalves) 154. 

 Of these, 29 belong to the land, 42 to fresh water, and 197 are 

 marine. 



The numerous discoveries which have been made in this 

 branch of our zoology, are not a little surprising, and indicate the 

 zeal with which the subject has been investigated. The catalogue 

 of shells in Professor Hitchcock's Report, of 1833, contained 

 126 species ; and that of 1835, which was made out with great 

 care, contained 165 species. Of course, there are now added to 

 the second catalogue J03 species, and to the first 142 species. 

 Of these, 70 have been discovered and described within the last 

 five years. In some genera the accessions have been remarkable. 

 Margarita, Pleurotoma, Cerithium, Bulla, and Nucula may be 

 specially mentioned. 



GEOGRAPHICAL' DISTRIBUTION. The geographical distribu- 

 tion of shells is a subject of increasing interest, and of some prac- 

 tical importance from its bearing on Geology. In this State we 

 have a remarkable instance of geographical limitation, and should 

 not pass it without notice. 



The land and fresh-water univalves are all distributed over 

 every part of the territory, with the exceptions of Helix horttnsis, 

 which is as yet confined to some parts of the sea-coast, and Helix 

 tridentala, hirsuta, and monodon, which are found only in the in- 

 terior and western portions. Of the fresh- water mussels we find 

 U. complanatus, radiatus, and probably nasutus, in every region ; 

 U. cariosus is only found in the Connecticut and its tributaries, 

 and in Plymouth ponds ; JInodon cataract a and Jllasm. arcudta 

 and margindta are found everywhere in the interior, while A'no- 

 don implicdta is perhaps entirely limited, in this State, to ponds 

 in Essex and Middlesex, and JInodon undid ida to Blackstone 

 river and its branches. 



The distribution of the marine shells is well worthy of notice as 

 a geological fact. Cape Cod, the right arm of \he Common- 

 wealth, reaches out into the ocean, some 50 or 60 miles. It is 



