338 ORTMANN— THE ALLEGHENIAN DIVIDE. [April 18, 



Chapter 4. 

 Summary of Distributional Facts which call for an Expla- 



TION. 



The above is the faunistic material which I have been able to col- 

 lect. Comparing the facts observed in the different groups of fresh- 

 water animals discussed, several classes have been brought to our 

 attention repeatedly, and they may be condensed under the following 

 generalized heads. 



I. Western Side. 



1. The AUeghcnian divide actually forms a sharp faunistic bound- 

 ary for a great number of freshwater creatures. This is most evi- 

 dent for the forms of the interior basin, which go up to a greater 

 or lesser distance in the upper Ohio drainage, but do not cross the 

 divide. To these belongs the bulk of the Najad-iauna. ; the genus 

 Pleurocera and the western species of Goniobasis, among the Pleu- 

 roceridce; at least one species of Campeloma (C. ponderosum) ; and 

 the group of Cambarus rusticiis and propinquus of the crayfishes 

 (which are closely allied). 



In a general way the interior basin fauna appears as a unit, a 

 number of species, chiefly N a jades, being found uniformly in all 

 parts of the Ohio drainage, from the upper Tennessee region to the 

 upper Allegheny River. 



2. Nevertheless there are indications of a differentiation into sev- 

 eral subdivisions, which may be described as follows: 



(a) The most sharply differentiated part is the upper Tennessee 

 region, and to this belongs probably the whole Cumberland-Ten- 

 nessee drainage. This is clearly seen in the Najades, in the Pleuro- 

 ceridcc, and in the existence of a peculiar species of crayfish, Cam- 

 barus spinosus, belonging to the rusticus group. 



(&) Another part comprises the main fauna of the Ohio, chiefly 

 of the middle and upper parts, and its tributaries. This fauna shows 

 preeminently the uniformity mentioned above, and goes from Lick- 

 ing and Big Sandy rivers in Kentucky to the upper Allegheny, in- 

 cluding the Kanawha and Monongahela. In the Allegheny this 

 fauna goes to the headwaters. But in the Kanawha and Monon- 



