GEOGRAPHICAL DISTRIBUTION. 179 



Group IV. are found here, while Groups I. and II. are represented by only -two species 

 each, C. Blandingii and pcllucidus, and C. simulans and gracilia. The southern province 

 contains thirty-six species, twenty-eight of which are not found beyond its limits. From 

 the northern province twenty-four species are known, sixteen of them peculiar to it. 



VII. In the territory occupied by the genus Cambarus the waters of the South and 

 West are richer in species than the waters of the Northeast. This will appear evident 

 on inspection of the table of distribution according to States, on page 165, or according 

 to river systems, on page 173. The well-explored New England States afford but one 

 species; Pennsylvania, four or five: while the less narrowly searched States to the 

 south and west yield much larger numbers; as Alabama, eleven; Georgia, thirteen; Ten- 

 nessee, twelve ; Indiana, ten. 



VIII. The crayfishes of the upper part of a river basin are often different from those 

 found in the lower part of its course,* even when the river does not traverse a great dis- 

 tance in latitude. The distinction between the species of the upper waters and those 

 of the lower waters is most marked in rivers that have a heavy fall from their source to 

 their mouth. In the upper waters of the Santee basin, for instance, 0. B/irtonii, lati- 

 nianws, aciuninatus, and spinosus are found; in the lower portion of the same basin live 

 C. Blandingii, var. acuta, and C. troglodytes. So with Astacus: the lower part of the 

 Columbia River, near its mouth, is frequented by A. leniusculus and A. Trowbrid-gii; 

 while above the Cascades A. Klamatlicnsis is found, and yet higher, in the head-waters 

 of the Snake Eiver in Idaho, A. Gambdii. 



IX. Distribution is often controlled by the character of the stream (temperature, 

 rapidity, purity, etc.) rather than by continuity of water communication. Thus, a species 

 of restricted range may be found in the upper waters of streams that rise in the same 

 mountain range, but flow in opposite directions and discharge at points far distant, and yet 

 be unknown in the lower portions of the same streams. For example, C. cxtraneus and 

 C. spinosus are found in the upper waters of the Santee, Alabama, and Tennessee Paver 

 systems. This fact is more easily explained in the case of crayfishes, many of which pos- 

 sess a singular faculty for living a long time away from the water, than in the case of 

 fresh-water fishes, where the same phenomenon of distribution has been pointed out by 

 Cope and by Jordan.f 



* This was observed by Agassiz in the case of fishes and mollusks. See his " Lake Superior," p. 247, 

 Boston, 1850. 



t See Cope, Journ. Acad. Nat. Sci. Phila., New Series, VI. 207 et seqq., and Jordan, Bull. U. S. Nat. 

 Mus., No. 12. 



