ORTMANN: THE CRAWFISHES OF THE STATE OF PENNSYLVANIA 505 
(a) C. limosus. 
It has been repeatedly emphasized above, as well as in a previous paper, that 
this species is well isolated morphologically and geographically. I have introduced 
it as one of the examples for the rule that ‘“ morphologically isolated species occupy 
isolated stations’ (Ortman, 1905), p. 127), and also for the rule that “discon- 
tinuity of distribution is a proof of antiquity ” (ibid.). Both rulesare beautifully illus- 
trated by this species, particularly in contrast to the other rule that ‘closely allied- 
species occupy neighboring areas.” Thus not only the effect of isolation, as produc- 
ing species, is evident in C. limosus, but it is‘also seen that the degree of isolation 
is in direct proportion to the sharpness of the expression of the specific characters. 
C. limosus is geographically the most sharply isolated species of our crawfishes, its 
area being several hundred miles distant from that of the most closely allied forms in 
Indiana and Kentucky. Correspondingly it is also morphologically well marked, 
being sharply distinguished from the other Pennsylvanian species, as well as from 
species in the west which are closely related to it. Isolation in this case is purely 
topographical, since the ecological habits of C. indianensis seem to be similar, (Hay; 
1896, p. 495); though another allied form, C. sloani, differs slightly ecologically, 
(Faxon, 1885, p. 90). 
(b) OC. propinquus, C. propingquus sanborni, C. obscwrus. 
These three forms, as far as our present knowledge goes, are sharply separated 
topographically, while they agree with each other ecologically, but the topograph- 
ical boundaries between them are not everywhere uniformly sharp. In fact, the 
ranges of these three forms are connected on the one side by the Ohio River, on the 
other side by the basin of the Great Lakes. 
As we have seen above, the present connection of these forms is a secondary 
feature developed during the latter part of the Glacial Period, while anterior to this, 
at the beginning of the Glacial epoch, different conditions prevailed, which were 
different in turn from those of still earlier times. ‘The history of these forms was 
probably as follows. At the end of the Tertiary a form corresponding to these three 
crawfishes existed in the drainage of the Erigan River. Probably there was only a 
single species resembling the present C. propinquus. This species lived in the Eri- 
gan River, as well as in its southern tributaries, and there was no chance for it to 
split up into different species, although variations may have occurred. When the 
advancing ice of the Glacial Period covered the Erigan River and thus separated 
the southern tributaries from each other, the latter formed Jakes, and later, by over- 
flow (or other means) they were connected again. Thus the present Ohio was 
created. The temporary isolation of these rivers at the beginning of the Glacial 
