ORTMANN: THE CRAWFISHES OF THE STATE OF PENNSYLVANIA 431 



An alternative supposition might be entertained. C. limosms might have arrived 

 ill its present home coming from the south by way of the Atlantic Coastal Plain. 

 This, however, does not seem probable. First of all, the distribution of V. limos-us 

 does not extend southward beyond \'irginia, and even in \'ir.<,'inia it is known only 

 from a fijw localities. Southward no representative of this group is known on the 

 coastal plain, and, if C. limosus had come from the south, traces of this migration 

 might be expected. On the other hand, if it came from tbe nortli, as we here 

 assume, the fact that it did not spread beyond \^irginia may be accounted for by the 

 presence of another group of this genus, the subgenus (Aimharus (hlandingi-sectlon), 

 in the soutbern parts of the coastal plain, which, like C. Umosu.-^, prefei-s ponds and 

 sluggish streams. Indeed both species ( C. limosus and C. blandiugi) are found actually 

 associated at the same localities (by Faxon, 1885«, p. 88, at Trenton, New Jei-sey, 

 and by the writer in the Delaware and IJaritan Canal at Princeton, New Jei-sey), 

 but we must bear in mind that in New Jersey, and also in Maryland and Virginia, 

 C. hiaiidingi is an intruder, its chief domain being in the Carolinas. 

 The following are conclusions from the above considerations : 

 Cambarus Ihno>(US is an ancient species, characterized by morpiiological and 

 geographical isolation. The most closely related forms are found in Kentucky and 

 southern Indiana. An attempt to explain the presence of C. limosii.s at its present 

 location has to connect its range with that of these related species. A connection 

 by way of the Atlantic Coastal Plain southward is out of the question. Thus only 

 the connection across the Appalachian system remains. The fact that the rivere 

 just west of the mountains in western Penns3dvania are occupied by a more 

 advanced group of species (^;ro/m?r/f( »,v-group) of a subgenus which is cei^tainly of 

 Preglacial age, as we shall see below, leads us to the conclusion that the liruotuii- 

 group also must be not only Preglacial, but older than the propmquu,s-group. But 

 at that time there was no direct way from the lower Ohio, where its center of dis- 

 tribution was situated, into western Pennsylvania and across the mountains, the 

 Ohio having no existence as yet, and the general drainage in this region being to 

 the north. This leads us to assume a former more northern range of tbe limnxus- 

 group, extending into Preglacial Canada ; and this assumption furnishes an expla- 

 nation why it was possible for the Olacial Epoch to cut the range of the llmosiis- 

 group in two, leaving no representative of it in the region now drained by the 

 middle and upper Ohio. Carabunis liravsua is a Tcrtinri/ tijpc, and it reached its pres- 

 ent area coming from the west and by way of the north, being driven so^ith along the 

 Atlantic Coastal Plain by the advancing ice of the Glacial Periml. It .survived during 

 the Glacial Period in the region of the lower Delaware River and Chesajwah: Hay. while 



