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THE AMERICAN NATURALIST. [Vol. XXXVII. 



but in the crayfishes the problem is not a simple one since the 

 animals may remain out of water for a considerable length of 

 time and it is to be expected that the conditions under which 

 they may be found are not always the same. C. virilis usually 

 occurs in running streams but may also be found in stagnant 

 ponds with C. immunis and C. gracilis and has been known to 

 resort to burrowing. C. bartonii seems to be characteristic of 

 the cooler mountain springs and streams, but it is also found in 

 limestone caves with C. pcllitcidus and associated with C. dioge- 

 ncs, a burrowing species, with the same habits. C. affinis and 

 C. blandingii may also be used as illustrations of the same point. 



While an examination of a table of distribution of species 

 offers some interesting suggestions, it is as yet impossible to 

 prepare a list of species characteristic of certain types of local- 

 ities, though many species may be accurately assigned. 



Several species appear to be confined to mountain streams. 

 The crayfishes of the Ozark Mountains are little known, but the 

 point is illustrated in the Appalachian tributaries of streams 

 emptying into the Atlantic Ocean and the Gulf of Mexico. The 

 lower courses of these streams are very different from their 

 sources and the general Fauna is very different. C. cxtrancus, 

 C. spinosns, C. bartonii, C. acnminatus, C. forceps, C. longulus 

 and other species seem to be confined for the most part to 

 mountain streams. As is the case in fishes, the same species 

 may occur in the head waters of streams originating on opposite 

 sides of a divide and debouching at widely separated points. 

 Some species found in the lowland portions of the same streams 

 are characteristic so far as yet known, and some idea of the 

 species characteristic of the lower portions of the course of a 

 stream — the lowland forms — may be gained from an examina- 

 tion of a list of the forms reported from the Atlantic coast plain 

 and the lower portions of the Gulf States. It must be remem- 

 bered, however, that elevation above the sea is not the only 

 factor producing lowland conditions so far as faunal and floral 

 distribution is concerned. 



Faxon pointed out that the greater the fall of a stream the 

 greater the difference between the species of the upper and 

 lower portion of its course and while the question needs careful 



