V 



430 MEMOIRS OF THE CARNEGIE MUSEUM 



sylvania or New York, the advancing ice of the ({lacial Period must have entirely 

 covered a large part of this range. In the central parts, in Ohio and western Penn- 

 s^dvania, it was impossible for these forms to retreat southward, these parts being 

 occupied by another vigorous group of river-crawfishes, as we shall see Ijelow (/'/-o- 

 j>ivijtn(s-grtm\>), and onl_y in the east and west a chance to survive was left. The 

 eastern remnant is the present ('. limoi^ns, the western is the group of species found 

 now in southern Indiana and Kentucky. 



How C. liviKtSKs reached the Atlantic ("oastal Plain from the Elrigan l)asin is very 

 hypothetical. One suggestion may be made. Not only does the North Branch of 

 the Sus(]uehanna seem to be a reversed I'iver, but the West Branch has captured 

 a large jiart of the original drainage of the Alleghany Plateau in Potter, ( 'ameron, 

 and ('learfield Counties. Davis (1889, p. 248) believes that this happened largely 

 in Pretertiary times, since he tliinlcs that the Alleghany Plateau belongs to the 

 Cretaceous peneplain. However, Campbell (1903, p. 280) has shown that there 

 are two old base levels in northern Pennsylvania, an older one (Cretaceous), iden- 

 tical with that of Davis, and a younger one (1,000 to 2,200 feet) corresponding to 

 the Harrisburg peneplain of Old Tertiary age. k>ince the headwaters of the West 

 Branch of the Susquehanna are carved into this second peneplain, it is probable 

 tliat during Tertiary times the stream-piracy of the Susquehanna was going on 

 rather vigorously. If we assume that ('. Jiniosus in Tertiary time existed in this 

 part of the Erigan River drainage, namely in tlie Old Upper and Middle Alleghany 

 Rivers,^" which did not belong to the Old Monongahela or Spencer River, it must 

 have been possible for it to get into the Susquehanna drainage in consequence of 

 this stream-piracy in Tertiai-y times. This, however, is a mere suggestion. There 

 is no other evidence for it but the bare fact tliat stream-piracy has gone on in this 

 region. I mention it here only to show that the crossing over of this species into 

 the Atlantic drainage is not altogether unthinkable. 



After arriving in tlie coastal plain C. Umosus was cut off in the (dacial Period 

 from its allied forms in the west. But it survived, and in Postglacial times was 

 able to advance again. But the Postglacial dispersal cannot have amounted to 

 much, since the increasing roughness of the streams, caused by the Postglacial eleva- 

 tion of the country, was not favoi'al)le to a nortliward exjiansion. We do not know 

 the exact northern boundary of *". //mo.>--/(.y outside of our state. It is found in New 

 Jersey as far north as Morris ( 'ounty, yet we do not know whether it reaches Rari- 

 tan and New York 15ays, and the Hudson River. No positive record from New 

 York State is at hand (see De Kay, 18U, p. 23, and Pauhnier, 1905, p. 117). 



"See : Carll, 1880, pp. 333 and 33«, map, PI. 2 ; Leverett, 1893, pp. 129 and 132 ; Tight, 1903, map, PI. 1. 



