ORTMANN: THE CRAWFISHES OF THE STATE OF PENNSYLVANIA 437 
Leverett (1902, p. 89, Fig. 1) has given a map of the Old Monongahela River, 
which is reproduced on Plate XLII, Fig. 1, and alongside of it, Plate XLII, Fig. 2, 
I give a map of the present distribution of C. obscwrus. It is evident at a glance 
that there is close correlation between C. obscwrus and this old river. The most 
important features are furnished by the western boundary. The divide between 
the section of the Ohio which runs along the Panhandle of West Virginia 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 Ohio- just 
above New Martinsville (see Leverett’s map, p. 90, Fig. 2: ‘“‘ probable early di- 
vide”). As I have found (p. 434) this old divide coincides with the present boundary 
between C. obscurus and C. obscurus sanbormi. It also is significant that it is not the 
Tertiary (Preglacial) divide, which is located by Tight (1903, Pl. 11) just 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 by the fact that they belonged to different river systems; second, that we 
must assume the Preglacial age of the propinquus-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 others) ; 
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 Hrigan River drainage (either directly or by crossing a divide), of which it 
became characteristic.” It entered, consequently, also the southern tributaries of this 
river, and owing to the fact that there were three main tributaries, 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 group 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, and sur- 
vival here occurred. Both the Old Monongahela and the Old Kanawha were 
50 We have seen that a similar dispersion very likely took place in the case of the Jimosus-group. The latter being 
more primitive, we must assume that it formed a first and earlier wave of immigration from the Lower Ohio into the 
Erigan drainage, while the propinguus-group came later. This movement is still going on. There is evidence of a sub- 
sequent Postglacial wave (later than the Postglacial migration of C. propinquus) also starting from the Lower Ohio, and 
represented by the rusticus-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. 
