GEOGRAPHICAL DISTRTBTTTION. ;I37 



observations. Ten years since, we observed great num- 

 bers of Sellx liortensis on a small uninliabitcd island 

 comprising less than an acre of surface, near the shore 

 of Cape Ann, in Massachusetts. Dr. Mighels has 

 observed the same species very abundant on an island, 

 of only a few rods in extent, in Casco Bay. Helix sep- 

 temvolva and Helicina orhicidata occur in such quan- 

 tities as almost to cover the ground upon small islands 

 on the coast of Florida. The diminutive islands called 

 the Brothers, in Lake Champlain, near Burlington, fur- 

 nish the shells of that district in abundance. Oak 

 Island,' a little wooded islet a few miles eastward of 

 Boston, surrounded by extensive salt marshes and at 

 high water by the sea, was, a few years since, covered 

 by myi'iads of Bidimus lubricus, and Vertigo ovata. 

 And very recently, on throwing a bridge from Goat 

 Island, at Niagara Falls, to an islet near it, the surface 

 of which measures but a few hundred square feet, and 

 which had been previously inaccessible, it was found to 

 contain the Helices and Sucdnece of the neighborhood 

 so plentifully, that hundreds might have been taken in a 

 few moments. In all these cases, the fact that individ- 

 uals were greatly multiplied above their numbers on 

 the adjacent main land, was striking and beyond doubt. 

 The locality, in each instance, being situated within the 



1 The railroad from Boston to Salem now passes through this island, and 

 connects it, in two directions, with the main land. This will probably eflect 

 an entire change in its peculiar molluscous fauna, or rather cause the ex- 

 tinction of these animals. 



VOL. I. 17 



