ORTMANN : THE CRAWFISHES OF THK STATE OF PENNSYLVANIA 437 



Leverett (1902, p. 89, Fig. 1) has given a map of the Old Monongahehi River, 

 which is reproduced on Plate XLII. Fig. 1, and alongside of it. Plate XLII. Fig. 2, 

 I give a map of tlio present distrihution of ('. ubscKnis. It is evident at a glance 

 that there is close correlation between ('. (Mairus and this old river. The most 

 important features are furnished 1)\- the western boundary. The divide between 

 the section of the Ohio which runs along the Panhandle of West \'irginia and the 

 Muskingum-Tuscarawas drainage is the old divide between the drainages of the 

 Spencer River and the Old Kanawha. This divide crosses the present C)hi(j just 

 above New Martinsville (see Leverett's map, p. 90, Fig. 2 : " proljable early di- 

 vide"). As I have found (p. 434) this old divide coincides with the present boundary 

 between C. obscurus and C. obscurus scinborni. It also is significant that it is not the 

 Tertiary (Preglacial) divide, whidi is located by Tight (1903, PI. 11) ju.st below New 

 Martinsville, nor the "later divide" of Leverett (/. c), but just the one which 

 existed at the beginning of the Glacial Period. We shall have to return to 

 this topic. 



Thus it is clear, first, that the original separation of these two forms was brought 

 about Ijy the fact that they belonged to different river systems; second, that we 

 must assume, the Preglacial age of the jii'opinquus-group ; and third, that the distri- 

 bution of these crawfishes furnishes additional evidence for the correctness of the 

 view of the Old Monongahela and Old Kanawha, as held by Leverett (and otliei"s) ; 

 and with reference to these crawfishes it seems to me that the following theory is 

 rather well founded. 



In Preglacial times, the propinquus-group, coming from the southwest {lower Ohio) 

 reached the Erigan River drainage (either dircctlg or bij crossing a divide), of which it 

 became characteristic.^ It entered, consequently, also the southern tributaries of this 

 river, and ojving to the fact that there were three main irilnUaries, this group developed 

 the tendency to split up into as many geographical forms. These were apparently the 

 conditions when the Glacial Period began. 



The chief effect of the advancing ice was that the northern parts of the range of 

 this gi'oup were covered by ice. Only in the region of the headwaters of these 

 rivers, to the south of the edge of the ice, was there a chance to survive, anil sur- 

 vival here occurred. Both the Old Monongahela and the Old Kanawha were 



'*' We have seen that a similar dispersion very likely took place in the case of the limu.iuanTOup. The latter beinj; 

 more primitive, we must assume that it formed a lirst aud earlier wave of immigration from the Lower Ohio into the 

 Eri(;an drainage, while the proyji'iijiiKs-group came later. This movement is still goinR on. There is evidence of a sob- 

 seqnent Postglacial wave (later than the Postglacial migration of C. pnipiiitiuiif) also sUrting from the Lower Ohio, and 

 represented by the nis^ciw-group, which again has all the morphological marks of a more recent type than the propin- 

 quus-group. But this is outside of the scope of the present paper. 



