462 MEMOIRS OF THE CARNEGIE MUSEUM 



origin of distribution, and in that case C. diogcncs would also have come from the 

 north, being driven back by the advancing ice. 



This necessitates the further supposition that C. (lUxjenes is a Preglacial species 

 (another reason for this has been mentioned above), which extended before the be- 

 ginning of the Glacial Period further north,, prubaldy from western Penns^dvania 

 into New York or even beyond. This is not improbable, since the Alleghany 

 Plateau stretched considerably to the north in Preglacial times (see Powell, 1<S!)(), 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 m^rthern 

 extremity of the Appalachian sj^stem. The coming of the ice must then have i-e- 

 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 iiorfJt- 

 easfcni biota, but its dispersal in Postglacial times does not entirely agree with that 

 of the second ivavc. Indeed there is a slight indication of a northward adv^ance in 

 Pennsylvania, Michigan, AVisconsin, and ^linnesota, l)ut the main direction was 

 ivestward 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 oidy 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. Suvimar;/ of the Stuelics on Geographical Disfrihiition. 

 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 chiefiy the casein 

 instances where our knowledge of the extralimital distribution is defective. It is to 

 be hoped that similar investigations outside of our state may furni.^h additional evi- 

 dence to substantiate our conclusions, or if neccssar}', to modify them. This much, 

 however, is evident, that tlie 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. (Jn the accompanying map 

 (PI. XLIII), we notice the following particulars. In the eastern part of Penn.syl- 

 vania, along the Delaware River from Trenton, N. J., to Marcus Hook, Delaware 



