456 MEMOIRS OF THE CARNEGIE MUSEUM 



north of the Allegheny in the region of Claremont. At the same place thei'e is 

 another terrace north of the river, only 150 feet high, and consequently belonging 

 to a later period, so that here the final shifting of the river to the south took place 

 later than in the other cases. 



These facts, if they at all influenced the distrihutiim of ('. oiiondui/alciiKis, and I 

 believe they did, give us a hint as to the time of the immigration of this species. 

 The shifting of the rivers must liave taken place sometime dining the < dacial 

 Period, for we know that during this time a considerable amount of erosion was 

 accomplished, the 900 feet level belonging to the beginning (jf the Glacial time 

 (Lake Monongahela stage). ''- 



This leads us to the conclusion that C. nioibongaiensis must have l)een present in 

 this region during the Glacial Period, at least during a part of it, and shortly before 

 these channels were changed."^ It is quite possible that this species had its preserve 

 in Glacial time not far from the edge of the ice in southern Pennsylvania and 

 northern West Virginia, and that it began to advance as soon as the ice of the \A'is- 

 consin stage began to recede. This would fully explain the fact that this species 

 was able to cross first the Youghiogheny and Monongahela by the help of the west- 

 ward shifting of these rivers, thus opening a way into eastern Allegheny and M^est- 

 moreland Counties, and that it later crossed the Alleghany River at Claremont, 

 when its channel was changed to the present more southern position. 



The question remains, why C. mononyalemis, having crossed the Allegliany, 

 did not advance further north. It is found at Claremont (near Squaw Run), in 

 a comparatively restricted locality, which is not altogether favorable, being at a 

 rather low elevation. It has not l)oen able to I'each more favoi-al)le locations at 

 higher levels, the ascent being more or less difficult on account of the very precipi- 

 tous hillsides, and moreover it may not prosper here because of the presence of the 

 competing species, C. dUxjenrs, which is quite al)undant in this region. C. monon- 

 galevsis is here, so to speak, cornered, and surroimded by unfavorable physical, 

 ecological, and biocenotic conditions. 



If this species existed in this region during Glacial times its Preglacial origin 



*'The rivers were cut down even deeper tban they are at present, but the valleys were tilled up again, 100 feet or 

 more (see Jillsou, 1. c. ). According to Foshay ( 1890, p. 402), the chief erosion falls into the end of the Tertiary ; but 

 the presence of glacial material in the old river channels, 900 feet high ( East Liberty. Pittsburgh), places the deepen- 

 ing of the valleys at a later period. Possibly it was connected with and subsequent to the draining oft of Lake Monon- 

 gahela (Wnite, 1896, p. 3751. which happened sometime during the Glacial Period. Hice (1903, p. •-29) places this 

 cutting down below the present channels between the Kansan and the Wisconsin stages. 



'^The same conclusion is reached when we consider that this species cannot have immigrated before the present 

 Ohio was formed, that is to say, shortly before the Wisconsin stage (see last footnote). Otherwise the Ohio would not 

 form its western boundary. 



