460 MEMOIRS OF THE CARNEGIE MUSEUM 
and Kentucky. Thus it is difficult or impossible to arrive at any conclusion as to 
its center of dispersal. But at this point certain morphological observations may 
possibly afford some help. We have seen (p. 407) that in western Pennsylvania the 
areola is often not entirely obliterated, a condition which is certainly more primitive 
than the normal one. Such specimens are quite frequent in southwestern Pennsy]l- 
vania, while in the other parts of the range they are rather rare or entirely absent. 
This fact, according to Adams’ (1902, p. 122, 125) eighth criterion for the determi- 
nation of centers of dispersal, points clearly to southwestern Pennsylvania. Here 
the character of the areola is the least progressive, while in either direction from 
this center, to the east and to the west, it is more progressive. This conclusion is 
further substantiated by Adams’ seventh criterion: “location of least dependence 
upon a restricted habitat.” We do not know much about the “habitat” of C. diog- 
enes in the west and south, but it is certain that in western Pennsylvania it is less 
restricted than in eastern Pennsylvania. Along the Delaware River I found it ex- 
clusively in the black muck of the alluvial flats, while in western Pennsylvania it 
has a much wider range ecologically, being found in clay bottoms, on hillsides, near 
springs, swamps, and even on sandy or gravelly soil. 
Judging from these facts, and also from the general rule which holds good for 
the subgenus Bartonius, that its center is in the Appalachian region, we may safely 
assume that C. diogenes did not have its center on the Atlantic Coastal plain, nor in 
the western parts of its range in the Mississippi basin, but that it is somewhere on 
the Alleghanian Plateau; and since southwestern Pennsylvania and northern West 
Virginia are the only parts of this plateau occupied by this species, we have to place 
its center here. 
Here, as we have seen, it dwells chiefly upon the late Tertiary base-level of the 
Old Monongahela drainage, and I believe this was its original habitat. We have 
no means to decide whether it was already present in this region in late Tertiary 
times; but the simple fact that it does occupy an area, the physiographical features 
of which have developed in Tertiary times, is in favor of this assumption. _ Further 
on we shall become acquainted with another reason for this view. In the Ter- 
tiary period its range very likely extended further north; but the Glacial Period 
must have restricted it, and its preserve was in the region indicated. In Postgla- 
cial times it spread northward again, at least in Pennsylvania. Unlike C. monon- 
galensis, the rivers did not form a barrier, for this species largely descended into the 
valleys, going down to the river-bottoms and the very banks of the river,” and thus 
°5Tt is found frequently on islands in the rivers ( Neville and Twelve Mile Islands, near Pittsburgh). I have seen 
chimneys on the river banks near Verona, and obtained specimens on the banks of the Kiskiminetas at Kiskiminetas 
Junction. 
