460 MEMOIRS OF THE CARNEGIE MUSEUM 



and Kentucky. Thus it is diliicult 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 Avhich is certainly more i)rimitive 

 than the normal one. Such specimens are quite frequent in southwestern Pennsyl- 

 vania, while in the other parts of the range they are rather rare or entirely absent. 

 This flict, according to Adams' (1902, p. 122, 125) eighth criterion for the determi- 

 nation of centers of dispersal, points clearly to southwestern Pennsylvania. Hei'e 

 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 Ijy Adams' seventh criterion : " location of least dependence 

 upon a restricted habitat." We do not know much about the " habitat" of ('. dioy- 

 cHf.v 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 Hats, while in western Pennsylvania it 

 has a much wider range ecologically, being found in clay l)ottoms, 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 Bartoni'iis, 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 l:)y this species, we have to place 

 its center here. 



Here, as we have seen, it dwells chiefly upon the late Tertiary Ijase-level of the 

 Old Monongahela drainage, and I believe this was its original habitat. We have 

 no means to decide whether it was alread}' 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 (jJIacial 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 



"It is found frequently on islands in tlie 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. 



