376 ORTMANN— THE ALLEGHENIAX DIVIDE. [April i8, 



brier, it did not go beyond this drainage on the western side and 

 did not reach upper Tygart and Cheat as Anciilosa did. The rea- 

 sons for this as well as for the fact that it did not become specifically 

 distinct in New River are unknown for the present, but probably 

 they are to be found in a diiTerence of the time of migration from 

 that of Ancnlosa. 



From New River, C. longiilus got into James River by the same 

 means as Syuiphynota tappauiaiia and Anciilosa, i. e., by stream 

 capture. It did not get out of this drainage except at one place, in 

 the uppermost Shenandoah. This is probably to be connected with 

 the stream piracy committed by the Shenandoah all along its present 

 valley (see above, p. 347). Just at Waynesboro there is a wind gap 

 in the Blue Ridge, Rockfish Gap, which undoubtedly once served as 

 an outlet for a tributary of the James River (Rockfish Creek or 

 Mechum River), which was beheaded by the Shenandoah exactly 

 as was Beaverdam Creek at Snickers Gap (Davis, 1891, p. 576). 



The question remains, why C. longuhis did not spread over the 

 rest of the Shenandoah and Potomac drainage. This may be due 

 to ecological causes. The species may not find farther down in the 

 Shenandoah a congenial environment. Where I found C. longuhis 

 the water was always rough and full of rocks, and the lower Shen- 

 andoah, although by no means a sluggish river, has considerable 

 quiet stretches. I also found this species generally at elevations 

 higher than the Shenandoah in the average. This would correspond 

 to a degree to the conditions seen in C. hartoni, which is also a spe- 

 cies avoiding larger streams and quiet water. 



Taking these last three cases together, Symphynota tappaniana, 

 Anciilosa, and Camhanis longuhis, it is seen that, although they 

 differ in particulars, they fall under one general head, and that very 

 likely similar causes were working to effect their distribution. Dis- 

 regarding Strophitns and Alasmidonta, which probably crossed the 

 divide farther north, they are the only cases where freshwater forms 

 seem to have crossed the Allegheny divide in its central parts, prob- 

 ably by the help of stream capture. 



The total number of such cases is very small compared with the 

 numerous cases which follow the general rule, that the Allegheny 



