370 ORTMANN— THE ALLEGHENIAN DIVIDE. [April 18, 



ent,. the absence of it in the New River system is the most important 

 fact which speaks for the assumption made above. 



Alasiiiidonta iimrgiiiata and Alasuiidonta inargitiata susquclmnncu. 



The typical western Alasmidonta marginata has a wide distribu- 

 tion in the interior basin, and in the Allegheny Mountains it goes 

 up into the headwaters of the Holston, Clinch, into New River, and 

 into the uppermost Allegheny River, but it is not found in the head- 

 waters of the mountain-tributaries of the Monongahela (although 

 it is immediately below the canyon in the Cheat). In the upper 

 Allegheny, it goes, like Strophitus, into very small streams,'-' and it 

 is in general a species characteristic, for smaller streams, avoiding 

 large rivers. 



On the Atlantic side, it is represented by two forms. The one 

 is Alasmidonta vavicosa, a closely allied, but nevertheless sharply 

 distinct species, which has been discussed above (p. 363 f.) together 

 with those forms constituting the northern element in the Atlantic 

 fauna, which migrated, in Preglacial times, around the northern end 

 of the Appalachian chain. 



But there is a second representative on the Atlantic side, which 

 has been hitherto overlooked, and which I have called Alasmidonta 

 marginata susquehanncc, which stands much closer to the western 

 form, in fact, is very hard to distinguish from it. This form is re- 

 stricted to the Susquehanna drainage in Pennsylvania and New York, 

 and it is found frequently associated with A. varicosa, but is always 

 perfectly distinct from it. 



It seems, according to the material at hand, that Alasmidonta 

 marginata susquehannce has its metropolis in the Juniata River and 

 the part of the Susquehanna in central Pennsylvania, which is below 

 the junction of the west and north branches. It has not been 

 found in the west branch and its tributaries (although Al. varicosa 

 is there), but we should consider that the fauna of this branch is 

 poorly known, and that it has been largely destroyed by pollution 

 from mine waters. 



"' Allegheny River, Larabee, McKean Co. ; Little Mahoning Creek, Good- 

 ville, Indiana Co. ; Loyalhanna River, Ligonier, Westmoreland Co. ; Quema- 

 honing Creek, Stanton's Mill, Somerset Co. ; all in Pa. 



