294 DISTRIBUTION AND A'TIOLOGY 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. 
8. The gradual subsidence of the posterior part of 
