OUTMANN : THE CHAWFISHES OF THE STATE OF PENNSYLVANIA 505 



(a) C. limosus. 



It has been repeatedly emphasized above, as well as in a prev'ious paper, that 

 this species is well isolated morphologically and geographically. I have introduced 

 it as one of the examples for the rule that " morphologicall}' isolated species occupy 

 isolated stations" (Oi'tnian, 1905/;, [). 127), and also for the rule that "discon- 

 tinuity of distribution is a proof of antiquity '' {ibid.). Both rulosare 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. Ihnoms, l)ut 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 w( st wliicli are closely related to it. Isolation in this case is purely 

 topographical, since the ecological habits of C. indiimenais seem to be similar, (Hay. 

 l(Si)6, p. 495) ; though another allied form, C. sfoani, differs slightly ecologically, 

 (Faxon, 1885, p. 90). 



(b) C. proinnquiis, C. froinnquus sanborni, C. obscuruK. 



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 ja-esent 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, diiferent conditions prevaileil, 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 tliree 

 crawfishes existed in the drainage of the Erigan Uiver. Probably there was only a 

 single species resembling the present C. j)rnpin<f(u.-<. This .species livf(l in the Kri- 

 gan River, as well as in its southern tributaries, and there was no chance for it to 

 split u{) into different species, although variations may have occurred. When tin- 

 advancing ice of the Glacial Period covered the Krigan River and thus separated 

 the southern tril)utaries from each other, the latter formed lakes, and later, by over- 

 flow (or other means) they were connected again. Thus the present Ohio was 

 created. The temporary isolation of these rivei-s at the beginning of the (.ilacial 



