294 DISTRIBUTION AND AETIOLOGY OF THE CRAYFISHES. 



the older observers lumped all the Western European 

 forms which came under their notice under one species, 

 Astacus fluviatilis ; noting, more or less distinctly, the 

 stone crayfish and the noble crayfish as races or varieties 

 of that species. Later zoologists, comparing crayfishes 

 together more critically, and finding that the stone 

 crayfish is ordinarily markedly different from the noble 

 crayfish, concluded that there were no transitional forms, 

 and made the former into a distinct species, tacitly as- 

 suming that the differential characters are not such as 

 could be produced by variation. 



It is at present an open question whether further 

 investigation will or will not bear out either of these 

 assumptions. If large series of specimens of both stone 

 crayfishes and noble crayfishes from different localities 

 are carefully examined, they will be found to present 

 great variations in size and colour, in the tuberculation 

 of the carapace and limbs, and in the absolute and 

 relative sizes of the forceps. 



The most constant characters of the stone crayfish 

 are : 



1. The tapering form of the rostrum and the approxi- 

 mation of the lateral spines to its point ; the distance 

 between these spines being about equal to their distance 

 from the apex of the rostrum (fig. 61, A). 



2. The development of one or two spines from the 

 ventral margin of the rostrum. 



3. The gradual subsidence of the posterior part of 



