432 MEMOIRS OF THE CARNEGIE MUSEUM 
‘all the vest of the former range of the growp was covered by ice and its representatives were 
destroyed, with the exception of a small remnant in the southwestern portion of the range, 
in southern Indiana and Kentucky, 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 (Ohio, western Pennsylvania, 
and West Virginia), was that here the rivers were occupied by another group of 
the subgenus. 
The above is a mere theory, and it remains doubtful by which way C. limosus 
reached the Atlantic Coastal Plain. ‘The assumption that it was by way of the 
Erigan River and the St. Lawrence basin satisfactorily accounts for the facts, but 
this is the only point directly in favor it. However, the study of the distribution 
of C. limosus is not yet finished, since the actual boundaries of the distribution, 
chiefly 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. limosus we have a species which survived during Glacial times in a part of 
the Atlantic Coastal Plain which is well to the north, not far from the southern edge 
of the ice. Of course this forms a part 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, C. limosws cannot be considered as belonging to the tundral 
biota (Adams, 1905, p. 58), but it belongs very likely to the second wave (north- 
eastern biota), with a slight suggestion of the third wave (southeastern biota) (J. c., 
pp- 58 and 62). As Adams indicates, the first and second waves of Postglacial dis- 
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 Canada, while the third wave was more stable and did not spread so far north- 
ward. With regard to its geographical location during Glacial times, C. limosus 
should be classed with the northeastern biota ; and with regard to its stability in Post- 
glacial times, with the southeastern. 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 plain 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, Pl. 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 
Tertiary 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. 
