432 MEMOIRS OF THE CARNEGIE MUSEUM 



((// ihe rest of the fonner range of the group loas covered hg ice and ila reprcsentatices were 

 dedragcd, with the exception of n srnoJI remnant in the soiithivesteni- portion of the range, 

 in southern Indiana and Keiitncki/. outside of the glaciated area. The reason why 

 this group was destroyed in the glaciated area, and was not able to retreat 

 southward and to survive in the intervening parts ((^hio, Avestern Pennsylvania, 

 and West \"irginia), was that here the rivers were occupied hx another group of 

 the subgenus. 



The above is a mere theory, and it I'eniains doubtful by which way ('. Vnnosus 

 reached the Atlantic Coastal Plain. The assumption that it was by way of the 

 Erigan River and the St. Lawrence l:)asin satisfactorily accounts for the facts, but 

 this is the only point directly in favor it. However, the .study of the distribution 

 of C. liiiiosus is not yet finished, since the actual boundaries of the distribution, 

 chiefl}' to the north and south, are not positively known. But this does not con- 

 cern us at present, since they are not situated in the State of Pennsylvania. 



In C. JiiiKisus we have a species which survived during Glacial times in a part of 

 the Atlantic ("oastal Plain which is well to the north, not far from the southern edge 

 of the ice. Of course this forms a pai't of Adams' (1902, p. 121 ) .southeastern center 

 in its widest sense, lying at its northeastern extremity. Although surviving not far 

 from the edge of the ice, ('. lirnosus cannot be considered as belonging to ihe tuudral 

 l)iota (Adams, 10U5, p. 58), but it belongs very likely to the second wave (north- 

 eastern biota), with a slight suggestion of the third wave (southeastern biota) (/. c, 

 pp. 58 and 62). As Adams indicates, the first and second waves of Postglacial di.s- 

 persion had their glacial homes in very narrow belts parallel to the southern edge of 

 the ice, while the southeastern (and southwestern) biota covered in Glacial times 

 wide tracts of country. The second wave largely invaded the coniferous forest-belt 

 of ( 'anada, while the third wave was more stable and did not spread so far north- 

 ward. A\'ith regard to its geographical location during Glacial times, ('. limosus 

 should be classed with the norfJieaster)! hiota ; and with regard to its stability in Post- 

 glacial times, with the semtheastern. But we are to consider that a Postglacial north- 

 ward dispersion was rendered difficult in this case by the physiographical features 

 of the country. The coastal ])lain with its sluggish streams and stagnant ponds 

 disappears in northern New .Jersey, the uplands (Piedmont Plateau) reaching the 

 coast in the vicinity of New York Bay (see McGee, 1888, PI. 2) ; this did not offer 

 advantageous conditions for this species, and thus it remained within comparatively 

 narrow limits in a corner, into which it was pushed in Glacial time. C. limosus is a 

 Tertiar,g relic at the northern extremity of the coastal plain, which has not been able 

 to expand its area to any considerable degree in Postglacial times. 



