462 MEMOIRS OF THE CARNEGIE MUSEUM 
origin of distribution, and in that case C. diogenes would also have come from the 
north, being driven back by the advancing ice. 
This necessitates the further supposition that C. diogenes is a Preglacial species 
(another reason for this has been mentioned above), which extended before the be- 
ginning of the Glacial Period further north, probably from western Pennsylvania 
into New York or even beyond. ‘This is not improbable, since the Alleghany 
Plateau stretched considerably to the north in Preglacial times (see Powell, 1896, p. 
80), and although the Preglacial features are largely obscured in this region, it might 
have been possible for this species to cross over into the coastal plain from western 
New York to southern New England or northern New Jersey, skirting the northern 
extremity of the Appalachian system. The coming of the ice must then have re- 
sulted in the obliteration of the northern connection of the range, thus dividing the 
originally continuous area into a western and an eastern subdivision, 
According to Adams’ classification (1905, p. 58), C. diogenes belongs to the north- 
eastern biota, but its dispersal in Postglacial times does not entirely agree with that 
of the second wave. Indeed there is a slight indication of a northward advance in 
Pennsylvania, Michigan, Wisconsin, and Minnesota, but the main direction was 
westward from the Alleghany Plateau, and even southward. This is undoubtedly 
due to the ecological peculiarities of this species (chimney-builder), it having found 
no competition in the directions named. In the eastern section of its range a north- 
ward advance is hardly noticeable. Here the species is more restricted ecologically 
(apparently a higher specialization), favoring only the black mud of alluvial deposits, 
and this very likely prevented its northern expansion. However, its exact distri- 
bution in New Jersey is unknown. 
8. Summary of the Studies on Geographical Distribution. 
We have been able in the preceding studies to advance a theory for the dispersal 
of each of the Pennsylvanian species of crawfishes. It cannot be denied that in cer- 
tain points our ideas do not seem to be well supported, but this is chiefly the case in 
instances where our knowledge of the extralimital distribution is defective. It is to 
be hoped that similar investigations outside of our state may furnish additional evi- 
dence to substantiate our conclusions, or if necessary, to modify them. This much, 
however, is evident, that the facts of the distribution of our species are due to two 
causes : (1) partly to the existing physiographical features of the country ; (2) partly 
to past conditions, which have now disappeared. On the accompanying map 
(Pl. XLII), we notice the following particulars. In the eastern part of Pennsyl- 
vania, along the Delaware River from Trenton, N. J., to Marcus Hook, Delaware 
