320 Cincinnati Society of Natural History. 



earlier want of knowledge in regard to distribution and variation as 

 applied to them. 



One remarkable fact should not be forgotten; that we begin with a 

 prevalence of ^72o^o??^cf5 at the east and northeast, which continues 

 across the northern part of the western distribution, while the southern 

 part finds these forms largely outnumbered b}' the Uniones, but with 

 the Anodontas re-appearing again in the west as the most representative 

 forms; and this statement, here referring to that portion of the drainage 

 north of the Ohio, is much more apparent throughout the rest of the 

 area now under consideration. If, now, Ave consider the shells of the 

 eastern slope of the Rock}' Mountain plateau, and the plains of the 

 Mississippi, from the. north southward, through Missouri and Arkansas 

 into Texas, we shall find, all the way through, a predominance of Ohio 

 river types; and I seriously doubt whether there is a single species in 

 this whole range, outside of them, in anj' part of this wide drainage, 

 that is an3'thing more than a variet}' of easily recognized Ohio forms; 

 and these remarks especiall}' apply to the Texas shells, which cer- 

 tainly abound in local varieties of Ohio types. 



In summing up the evidence upon wliich this statement rests, it 

 should not be forgotten that even in those streams which present the 

 greatest number of species not found in the Ohio, and which ma^^ thus 

 be call.ed abnormal, the central group, that containing the largest num- 

 ber of species, is the group made up of t3'pical Ohio river foi'ms ; and 

 this remark applies, without exception, to everj^ stream throughout 

 this wide range, from Ohio westward to the limit of the Mississippi 

 drainage, and southward to the western borders of Texas. These re- 

 marks must mainly apply to the Unionidoi, as there has been a much 

 more limited westward distribution of the Strepomatidce, though the few 

 species hitherto collected from the western slope of the Mississippi 

 basin and Texas, are referable, with the possible exception of a single 

 species more nearl}^ allied to the ^Mexican fauna (?), to tj^pes east of 

 the Mississippi. 



If, now, leaving this portion of our field, we direct attention to that 

 part of the Ohio drainage which lies south of that river and east of the 

 Mississippi, remarkable changes at once begin to present themselves. 



The first and most important of these is the appearance of manj' new 

 species and several genera oi Strepomatidoi^ and the excessive differen- 

 tiation of certain forms, which, from their prevalence, ma}^ be regarded 

 as central or t^'pical, this introduction and differentiation beginning 

 before we have crossed the State of Kentucky, and continuing through 



