ORTMANN : THE CRAWFISHES OF THE STATE OF PENNSYLVANIA 439 



(Indiana) at a tin)e when tliis basin was entirely covered by ice further east, thus 

 being closed more or less to an immigration from the central parts of Ohio (drain- 

 age of middle Ohio), and being closed entirely to an immigration from eastern 

 Ohio and western Pennsylvania (drainage of upper Ohio). 



This explains why C. jrropinquus, which survived in southern Indiana, had the 

 first chance to spread northward and t(j enter tlie future Hunjn-Erie basin by way 

 of Lake Maumee. The subsequent stages of this lake (Lake W'iiittlesey, Lake Warren, 

 etc.), are all direct continuations in time of Lake Maumee, and so it is not astonish- 

 ing that C. irropinqxnLs, after the final establishment of the St. Lawrence drainage,*-' 

 is found all over this region, not only in the Lake Huron and Lake J^rie basins, but 

 also farther down, in Lake Ontario and the Lower St. Lawrence drainage. In the 

 occupation of this whole region C. propinqmis was not interfered with by the other 

 forms, since no opportunity was given to C. propinquus sanhorni and C. obscums, to 

 enter the Erie basin, the drainages of the middle and upper Ohio remaining perma- 

 nently changed to the southwest, away from Lake Maumee, a condition which ob- 

 tains, with very slight changes, up to the present time. 



However, C. propmqmi.s sanhorni as well as C. (Mcanis, have entered the Lake 

 Erie drainage. With regard to the first, it maybe sufficient to state that it is found 

 in Lorain County, Ohio, in rivers and creeks running into the lake, and this is ap- 

 parently due to a comparatively recent immigration under similar conditions as in 

 the case of ('. oJisciirns in Pennsylvania. 11 if latter species has been discovered by 

 the writer in Crawford and Erie Counties. Pa., in streams fiowing to Lake Erie, 

 associated with the Lake Erie form, C. j^rojnnquv.-^. Thus C. obscurns must have 

 crossed the divide between the upper Beaver (Shenango) River ami Alleghany 

 River (French Creek) on the one side, and Lake Erie (Conneaut and Elk Creeks) on 

 the other, and the question is by what means this was accomphshed. 



It is only natural that C. obxcurus, surviving during Glacial times in soutli- 

 wcstern Pennsylvania and West Virginia, migratetl up the drainage of the upper 

 Ohio, chiefly the P.eaver and AUegiiany Rivers, in Postglacial times, for after 

 the end of the Glacial Period this system formed a unit", and no .serious l>arriers 

 to the dispersal were, or ai-e, present. Thus it was easy for this species to go up 



" The cliange of the westward ilraiiiiifje to an castwarii took place toward the cud of tin' Olncial Perio«l, as rooii m 

 the ice recede<l far enough to nncover Ijike Ontario (l-ake Irmiuois), thii» permitting tlic water to dr .in olT iliroiijili tlie 

 Moliawk, and hiter throuKh the St. Lawrence. This was ncconipanied prolmhly l>y a depression of the land in the 

 Northeast, enlniiimtiiiK in the marine invn.sion of thcSt I^iwrencc valley (Clmmplninmihmcrgence). (SceGmlmn, 1»»1, 



p. 59 et srq. ) 



"As to the formation of the present Alleghany out of the former Lower, Middle, and Ipper Alleghany, see Lev. 



erett, 1902, p. V29 it siq. 



