CHAPTER VI. 
THE DISTRIBUTION AND THE ZTIOLOGY OF THE 
CRAYFISHES. 
So far as I have been able to discover, all the cray- 
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 many 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. M’Intosh, 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 ; + but the question whether 
their diffusion, and even their introduction into this 
* Moore. Magazine of Natural History. New Series, IIL., 1839. 
+ Thompson. Annals and Magazine of Natural History, XI., 1843. 
