OP coNcnoLOOY. 221 



INTEODUCTION, 



fpiie number of described species of terrestrial mollusca known 

 to inhabit within the limits of the United States is not far 

 from 275 species, and many of these have a geographical dis- 

 tribution almost co-extensive with our territorial limits. There 

 are, however, some peculiarly Southern and Californian groups 

 of more restricted distribution, while there are not wanting, in 

 our far Southern and South-western States, stragglers from the 

 peculiar faunas of Mexico and Cuba ; and even a few European 

 species have become domesticated with us. 



While all the species discovered and described in the United 

 States to this date are included in the following pages, a few 

 Mexican species are also added, generally as illustratious of 

 allied forms inhabiting an adjacent country, many of which 

 will doubtless be detected within our b'oundary when our 

 little-known contiguous territories are more fully explored by 

 the naturalist. A brief account of the geographical distribu- 

 tion of the species and of their habits is included under the 

 description of each genus. 



Id the description of the species, for the sake of conciseness, 

 many characters are omitted which are common to, and have 

 already been used in the description of any of the groups, 

 genera, or higher divisions in which the species is included, 

 and, therefore, no specific description can he regarded as en- 

 tirely complete in itself. It will, of course, be readily under- 

 stood that adult shells of perfect growth are alone selected for 

 these descriptions, and the same species, when immature, pre- 

 sents a very different aspect. Mesodon ahholahris, an animal 

 of which the adult is furnished with a shell having a reflected 

 and appressed lip and covered umbilicus, is, when young, fur- 

 nished with a sharp- edged lip, and the umbilicus is open. The 

 observer will soon, by a few comparisons, be enabled readily 

 to separate the mature from the young shells, as the latter 

 always present a peculiarly unfinished, fragile aspect. 



When the ascertained geogra-phical distribution of a species 

 is extensive, we have generally indicated merely the States 

 forming its boundaries. Its occurrence may be presumed 

 within all the included region. More particular localities are 

 only stated when the hitherto ascertained habitat of a species 

 is extremely local. 



Measurements are made in millimetres, 25 -= 1 inch. 



