GEOGEAPHICAL DISTEIBUTIOK 175 



evn bounds of California. From the neighborhood of San Francisco, California, comes 

 A. nigrcsccns, a species vvhicli 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 Ooualaska Island, Alaska Territory (lat. 53° 52' N.). 



On the west coast of Mexico, at Mazatlan, a Cambarus occurs, C. Montezumm ; also a 

 Parastacine at Colima. 



General Conclusions derived from the Facts known concerning the Geo- 

 graphical Distribution of Crayfishes. 



I. Tlie crayfishes of tlie 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, Parastacinie, from the crayfishes 

 of the Northern hemisphere (Europe, Asia, and North America), which form a second sub- 

 family, Potamobiime (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 Nortliern hemisphere, when they took possession of the fresh 

 ■waters. The distribution of the different genera of Parastacince in the Southern hemi- 

 sphere will be considered in the second part of this memoir. 



II. The crayfishes belonging to the subfamily Potamobiinc'B occupy four geographical 

 areas, viz. : — 



(1.) The eastern and central part of tlie Nortli American continent. This area em- 

 braces that portion of Nortli 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 tlie Pacific at a former period. 



(3.) A tract on the eastern side of Asia, including the Ainoor 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 f and a western Eurasiatic area. The two areas in Noith America are 

 in close juxtaposition at the Rocky Mountain divide, whereas the eastern and western 

 Eurasiatic crayfishes are sundered by a broad tract in Centi'al Asia whose waters are 

 wholly destitute of these animals, as far as known. 



III. (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 

 crayfishes form a natural group (Cambaroides, page 126), in which the characters of 

 Astacus and Cambarus are combined. 



* Proc. Zoolog. Soo. London, 1878. 



f Eurasia is the single eoutiueut artificially divided into Europe and Asia. 



