448 MEMOIRS OF THE CARNEGIE MUSEUM 



fJiogcvcs-gro\i\}). Jf this is tlie case it very likely extended in the Tevtiiiry at least 

 as far north as at present, but the advancing ice of tlie (llacial Period must have 

 driven it south again, and it nnist have survived in the mountains of Virginia, 

 West Virginia, and states further south. Possibly southern Pennsylvania formed 

 part of its preserve in Glacial times, for th.e peculiar preference of this species for 

 cold water admits this assumption. Be this as it mav. it is certain that after the 

 retreat of the ice this species advanced, occupying or reoccnpying the whole state 

 of Pennsylvania, and keeping on in its northwai-d migration, until finally reaching 

 its present range. 



This advance in a northeastern direction clearly agrees with Adams' tliird h.ujli- 

 ivai/ uf dlsj)ersal {1902, p. 123) along the Appalachian chain, and V. Intiioni also 

 belongs to the northeastern biota of the second Postglacial wave (1905, p. 58). What 

 is interesting in this case is that an a(|uatic creature follows here the main strike of 

 the mountains, independent of the drainage systems. I have })reviously called 

 attention to this fact (l'.)05/', p. 129), and have pointed out that this is rather the 

 rule with the subgenus Bartoirias (this has been oKserved already 1)V Faxon, 1885*7, 

 p. 179). There is not the slightest doubt that this peculiarity is connected with the 

 ecological laws governing this species. It lives generally in the region of the head- 

 waters of the streams, where the dendritic conformation of the drainage sj'stems and 

 their mutual interlocking favors frequent shiftirjg of the divides in consequence of 

 stream-piracy. 



Moreover, '''. hartoni is a form which habituall}' leaves the water. It is found 

 not oidy in small streams, but also in springs, often at places where there is a very 

 scanty suppl}^ of water, and this has fcM'ced it to often assume burrowing habits. 

 Like ('. ohsciirvs it is al)le to survive exposure to the open air for a consideral)le time, 

 provided the temperature is not too high, and thus it is easy to imagine that it may 

 cross over divides during rainy oi' cloudy weather, wandering from spring to S[)ring 

 in the mountains. ( )n the other hand, we see that C imrtoni is not entirely absent 

 from larger streams, and if once established in a small part of the drainage of a cer- 

 tain I'iver it may easily be distributed over the rest of it l)y simply following the 

 course of the stream. Thus it is not strange that this species has occupied the whole 

 of the state, and this uniformity of distribution is chieily i\\w to the fact that the 

 whole of I'ennsylvnnia is hilly or mountainous, oHering everywhciv favorable condi- 

 tions for this species. The genei'al dispersal is due to two cau.ses : first the ability of 

 this species to cross watersheds b}' active migration ; and second to stream-piracy, 

 which has apparently plaj'ed a considerable pait in its dispersal. 



it should be noted, as we have seen above, that the size of this species decreases 



