CHAPTEE VI. 



THE DISTRIBUTION AND THE iETIOLOGY OF THE 

 CRAYFISHES. 



So far as I have been able to discover, all the cra}^- 

 fishes which inhabit the British islands agree in every 

 point with the full description given above, at p. 230. 

 They are abundant in some of our rivers, such as the 

 Isis, and other affluents of the Thames ; and they have 

 been observed in those of Devon ; * but they appear to 

 be absent from manj others. I cannot hear of any, for 

 example, in the Cam or the Ouse, on the east, or in 

 the rivers of Lancashire and Cheshire, on the west. 

 It is still more remarkable that, according to the best 

 information I can obtain, they are absent in the Severn, 

 though they are plentiful in the Thames and Severn canal. 

 Dr. MTntosh, who has paid particular attention to the 

 fauna of Scotland, assures me that crayfish are unknown 

 north of the Tweed. In Ireland, on the other hand, 

 they occur in many localities ; t but the question whether 

 their diffusion, and even their introduction into this 



* Moore. Magazine of Natural History. New Series, III., 1839, 

 t Thompson. Annals and Magazine of Natural History, XI., 1843. 



