360 ORTMANN— THE ALLEGHENIAN DIVIDE. [April 18, 



northward dispersal, also of stream piracy in the mountains. In a 

 few cases, the latter probably was the prime factor in the dispersal, 

 chiefly in the case of Anculosa carinata. 



Thus there is no difficulty in admitting the possibility of the dis- 

 persal of the Atlantic fauna over more or less of the whole region. 

 The facts in the distribution of the Najades, as well as in the Pleu- 

 roceridce, and in the crayfishes support this assumption. But the 

 other fact, that certain forms of the Atlantic slope did not reach 

 a universal distribution, and were apparently obstructed in their 

 dispersal at certain points, needs further discussion. This is a more 

 difficult problem, but, as far as possible, it will be taken up below. 



Fact II., i, (b). 



Aside from certain species (Najades: Elliptio fisherianus, Ano- 

 donta Cataracta and implicata, Eurynia nasuta, Lampsilis radiata, 

 cariosa, ochracea, and the crayfish Cambaras blandingi) , which are 

 more or less typically species of the lowlands or the great rivers, 

 the fauna of the Atlantic streams is rather uniform, in each sys- 

 tem, from the Piedmont Plateau upward into the mountains, to near 

 the sources. (See list no. 23 of Najades, and also Goniobasis vir- 

 ginica, Anculosa carinata, Cambarus limosus.) That is to say, the 

 fauna does not deteriorate, or very little so, in an upstream direc- 

 tion. This differs strikingly from the conditions on the western 

 side, where a gradual decrease of the number of species toward 

 the sources is the rule, or where we even observe a sudden disap- 

 pearance of species at certain points in the mountain streams. 



The explanation of this fact is found, as I believe, in a general 

 physiographical character of the Atlantic streams, which is best ex- 

 pressed by their profile (see our profiles on PI. XIIL, and PL XIV., 

 fig. 1). We see that the profiles of the Atlantic streams are more 

 nearly normal (Abbe, 1899, p. 61, fig 3 ; of course we must dis- 

 regard the falls line at the eastern edge of the Piedmont Plateau). 

 This profile indicates comparative stability, with the slope steepest 

 at the headwaters, decreasing rapidly just below headwaters, and 

 then gently farther down. These streams are more mature than 

 those of the western side. On the eastern side, new cycles of ero- 



