GEOGRAPHICAL DISTRIBUTION. 175 
ern bounds of California. From the neighborhood of San Francisco, California, comes 
A. nigrescens, a species which apparently extends far north along the coast, as there are 
specimens in the U.S. National Museum said to have been taken at Fort Steilacoom, 
Washington Territory, and Oonalaska Island, Alaska Territory (lat. 53° 52’ N.). 
On the west coast of Mexico, at Mazatlan, a Cambarus oceurs, C. Montezume ; also a 
Parastacine at Colima. 
General Conclusions derived from the Facts known concerning the Geo- 
graphical Distribution of Crayfishes. 
I. The erayfishes of the Southern hemisphere (Australia, Tasmania, New Zealand, 
Feejee Islands, Madagascar, and South America) possess certain characters in common 
(given on page 2) which separate them as a subfamily, Parastacine, from the crayfishes 
of the Northern hemisphere (Europe, Asia, and North America), which form a second sub- 
family, Potamobiine (page 2). This was first pointed out by Huxley,* who suggests, in 
explanation of this fact in the distribution of the crayfishes, that their marine ancestors 
were already differentiated into a Parastacine type in the Southern hemisphere and a. 
Potamobiine type in the Northern hemisphere, when they took possession of the fresh 
waters. The distribution of the different genera of Parastacinze in the Southern. hemi- 
sphere will be considered in the second part of this memoir. 
If. The crayfishes belonging to the subfamily Potamobiinse occupy four geographical 
areas, V1z.: — 
(1.) The eastern and central part of the North American continent. This area em- 
braces that portion of North America which lies east of the Rocky Mountains, drained 
by the rivers that flow into the Atlantic Ocean, Hudson’s Bay, and the Gulf of Mexico, 
from Lake Winnipeg on the north to Guatemala on the south. It includes the island 
of Cuba. 
(2.) The western slope of the North American continent, or the area drained by the 
rivers that flow into the Pacific Ocean. In this area is included the basin of the Great 
Salt Lake, which probably drained into the Pacific at a former period. 
(3.) A tract on the eastern side of Asia, including the Amoor River basin and Japan. 
(4.) An area including the greater part of Europe, and extending into Western Asia 
so as to embrace the Aralo-Caspian basin. 
Thus we have an eastern North American and a western North American area, an 
eastern Eurasiatic | and a western Eurasiatic area. The two areas in North America are 
in close juxtaposition at the Rocky Mountain divide, whereas the eastern and western 
Eurasiatic crayfishes are sundered by a broad tract in Central Asia whose waters are 
wholly destitute of these animals, as far as known. 
Ill. (1.) The western Eurasiatic and the western North American crayfishes belong 
to the genus Astacus (page 125). They are closely related, the European species differing 
from the western North American species barely more than the latter do from each other. 
(2.) The eastern North American crayfishes (Cambarus, page 3) are generically distinct 
from the western North American and European species. (3.) The eastern Eurasiatic 
erayfishes form a natural group (Cambaroides, page 126), in which the characters of 
Astacus and Cambarus are combined. 
* Proc. Zodlog. Soc. London, 1878. 
+ Eurasia is the single continent artificially divided into Europe and Asia. 
