GEOGRAPHICAL DISTRIBUTION. 179 
Group IV. are found here, while Groups I. and II. are represented by only two species 
each, C. Blandingii and pellucidus, and C. simulans and gracilis. 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 175. 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, C. bartoni, lati- 
manus, acuminatus, 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. leniuscwlus and A. Trowbridgii ; 
while above the Cascades A. Klamathensis is found, and yet higher, in the head-waters 
of the Snake River in Idaho, A. Gambelii. 
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. extranews and 
C. spinosus are found in the upper waters of the Santee, Alabama, and Tennessee River 
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.t a 
* This was observed by Agassiz in the case of fishes and mollusks. See his “ Lake Superior,” p. 247, 
Boston, 1850. 
+ See Cope, Journ. Acad. Nat. Sci. Phila., New Series, VI. 207 ef segg., and Jordan, Bull. U. 8. Nat. 
Mus., No. 12. 
