139 



Tin: Uxioxio.K ok thk Ohio Kivkh. Ilv \l. Ki.lswokth Call. 



lAiisrKACT.I 



Tliero aw now rocoii'iiized in the freshwater niolliiscan fauna of Nurth America 

 more than one tlions^and representatives of tiie great family of l^nioniche, or fresh- 

 water mussels. A few of these forms, wiiieh constituti' a peculiarly well-marked 

 division of the family, occur in Mexico and in Central Amciica. Less than a score 

 of species are foimd in Canatla. The rest are [)eculiar to the Cnited States and, 

 for the greater i)art, are found east of the Rocky Mountains. .More than ninety 

 per cent, of all known forms are from the regions east of the Mississippi and south 

 of the Ohio Rivers. The center of distribution for the desci'ii)e(l southern forms is 

 the great central plateau region of Middle Kentucky and Tennessee, Western i^orth 

 and South Carolina, and Northern (ieorgia and Alabama. Within the area as 

 above limited, occur nearly all the species that are known — outside of the great 

 Unionidiv group known as the compldvdtiis division. In all the larger streams, 

 and in most ot the smaller, throughout all this region, the members of the family 

 flourish in both great nundjers of individuals and species. Al)out eighty per cent, 

 of all describeil North American forms come from this area, and some thirty per 

 cent, of all are from Tennessee, Alabama and Chattaiioochee Rivers, and their 

 tributaries. 



This singular, but interesting fact, has never yet received tlu' attention it 

 deserves, for geographic distribution, abundance in individuals, and diversity of 

 form are herein correlated clearly with certain geologic factcus. For instance, the 

 family is a very ancient one, and dates bat'k to I>evonian times at the latest. The 

 region under consideration has constituted a nniipie land-mass since a very early 

 period in the history of the continent. It has scarce been subjected to glaciation 

 — at least has not since the geologic rei-ord exhibited in its country rock began. 

 The very great diversity of form and the great abundance of these modern repre- 

 sentatives of a very ancient ty|)e, apjjcar i)lainly to be related in no small degree 

 to these factors. 



In investigating in this field, for some twelve years or more past, the s})ecies 

 and distribution of these mollusks, attention was necessarily directed to that pe- 

 culiar Unionine fauna which lies on the northern border of this area. This was 

 rendered necessary, in the first place, by the fact that the Ohio River had itself 

 furnished most of the earlier described types. The literature of the subject re- 

 veals some sixty species, distributed uneijually among the three I'nionine genera, 

 Unio, Anodunta and Mari/ari/aiia, and shows the forms distributed among these 

 genera iji an abundance which has the relation just given, viz.: Uiiio has the 

 greater nund)er of species and Mnniarlti'iia the least. 



