xlvi 



To the first of these forms undoubtedly belongs 

 Eiiryccdon, a new genus, which probably includes 

 more species than have yet been assigned to it; — 

 and Schizostoma. Of the six species of the former, 

 one is from the Holston, another from the Cumber- 

 land, and the balance from the tributaries of the 

 Alabama River. 



SCHIZOSTOMA. 



This genus, embracing twenty-six species, divided 

 into two distinct groups of nearly equal respective 

 numbers, inhabits the Coosa River only, and in this 

 limited space exhibits all the range of variation in 

 form, size and ornamentation, belonging to genera 

 which possess a more extended geographical distri- 

 bution. 



MESESCniZA 



Contains at present only the type species. It is a 

 ver}^ small, fragile shell, inhabiting the Wabash 

 River, and does not appear to be of mature growth.* 



GOmOBASIS. 



This very large and wddely-extended genus em- 

 braces over two hundred and fifty species — more 

 than half of all the Strq?omatidm — and includes 

 the only representatives of the family west of the 

 Rocky Mountains, or south of the United States. 



One species, beautifully ridged with sharp, revolv- 

 ing ribs — the G. 2)'>'osclssa, of Anthony — is re- 

 ported simply from northern Alabama. There are 

 eighteen tuberculate species; the heavy, compact 

 ones being principally from the branches of Alabama 

 River, while the elong-ated ones are found in the 

 Tennessee. 



In the latter is included a very distinct group, 

 typified by Postellii, of Lea, belonging to the 

 tributaries of the Tennessee, in Northwest Georgia. 

 Two or three allied species are found in Florida. 



* The validity of the genus is doubtful. No specimens have been 

 collected since the type series, and they all appear to have been injured. 



