OF CONCHOLOGY. 12Y 



Total in 1st section - - - 126 species, ) 



" 2d " - - - 307 " 1464 species in all. 



" 3d " ... 31 '•■ \ 



We find that, while some groups of species extend over a 

 very wide territorial space, other groups are extremely re- 

 stricted, and yet are frequently characterized by as great 

 variation in form, size, ornamentation, &c., as the former. 

 The Gonlohasic Group occupy the entire extent of our coun- 

 try, represented by the sole species of our Northern Atlantic 

 States, the very few forms of the great Northern Lakes and 

 the species of the Pacific States, while they also occupy the 

 entire southern country, with one or two species in Mexico 

 and Cuba. 



The Trypanostomoid Section, on the contrary, is very much 

 more restricted, being confined principally to the streams 

 tributary to the Mississippi and the Gulf of Mexico. The 

 Mississippi appears to form their western boundary. 



While the Trypanostomoid forms attain their maximum 

 development in size and number in the Tennessee Eiver, they 

 are, to a very great extent, replaced by the Goniobasic forms in 

 the Coosa River, which is undoubtedly the metropolis of the 

 latter. The most striking genus of each of these groups is ab- 

 solutely confined to the respective streams in which the 

 groups had their origin. Thus, lo and Schizostoma are inhabi- 

 tants, the first of the Tennessee and branches, the second of 

 the Coosa, and neither of them are elsewhere found. 



Assuming the Ohio River as a dividing line, we find that 

 ninety-five per cent, of all the species originate south of it. Even 

 a smaller proportion inhabit the rivers east of the Allegheny, 

 and west of the Rocky Mountains. In the west, no species of 

 Strepomatidse have been discovered in higher latitudes than the 

 northern boundary of the United States, while in the east, 

 the St. Lawrence River and tributaries, appears to be the 

 northern limit of the family. 



We thus find the Strepomatidse to be distributed almost 

 exclusively within the limits of the United States, a distribu- 

 tion co-extensive with our Viviparidve and other families of 

 Mollusca ; clearly indicating that our country constitutes a 

 distinct faunal province. For, as the Viviparidse are replaced 

 in Mexico by AmpuUaria, so, for the Strepomatidse, are sub- 

 stituted the more ponderous Pachycheili. Between the former 

 and the latter extend the broad plains of Texas, with rivers 

 devoid of species, forming a barrier to the intermingling of 

 the two groups. Besides this, the Mississippi River, from 

 the junction of the Ohio to its mouth, appears to have formed 

 a barrier to the westward progression of the Strepomatidse, 



