I. THi: CRAYFISH OF ALFEGHENY COUNTY, PA. 



By E. B. Williamson. 



With teachers an<l instructors the crayfish is a favorite type, both 

 for dissection and for ilhistration, and yet but little attention is given 

 to these interesting crustaceans from a systematic standi)oint. Even 

 among those who should be better informed the impression prevails 

 that there is but one species in this country, or at least in the eastern 

 United States, and that this single species l)elongs to the genus Asta- 

 ciis. As a matter of fact all the crayfishes in North America east of 

 the Rocky Mountains, in the area drained by rivers flowing into the 

 Atlantic Ocean, Hudson's Bay, and the Gulf of Mexico, and in the 

 Island of Cuba, belong to the genus Camhanis? One species of this 

 genus is found in the Yellowstone River. Otherwise all the s]iecies 

 west of the Rockies to the Pacific coast l)elong to the genus Asfaciis. 

 Huxley, in his paper "On the Classification and the Distribution of the 

 Crayfishes," Proceedings of the Zoological Society of Fondon, 1878, 

 pp. 752-788, has divided crayfishes into two groups, to which Faxon 

 has given the rank of subfamilies. These subfamilies are the Po/aino- 

 Iniiue, the crayfishes of the northern hemisphere, and the ParastaciiuT, 

 the crayfishes of the southern hemisphere. The crayfishes of the 

 northern hemisphere are naturally divided into two genera, Astacus 

 and Caiiibariis. The distribution of these genera is very interesting. 

 As stated above, in North America Cambariis occurs east of the Rocky 

 Mountains and Astacus west. In Eastern Asia and Japan the cray- 

 fishes belong to the genus Cauihanis, while in Europe and Western 

 Asia they belong to the genus Astacus. That is, if one should move 

 round the world on parallel 50 degrees north, for example, and should 

 start in I'^astern N. A. and then travel westward, he would successively 

 encounter Camlmrus, Astacus, Caiiiharus, and Astacus. This curious 

 distribution has been explained by supposing that in the ocean, which 

 lay to the north of both continents in past geological ages, both forms 

 had appeared, though not as well separated as at present. When the 



' Faxon, .\ Revision of the Astacidie. See also Hagen, Monograph of the Nortli 

 American Astacidre. 



