THE GENERAL NAME, GENUS. 947 
the free end of the latter and being nearly parallel with it: 
In the first appendage (C), the anterior rolled edge (a) 
more closely embraces the posterior (b), and the groove 
is more completely converted into a tube. 
Tt will be observed that the differences between the 
English and the Californian crayfishes amount to ex- 
ceedingly little; but, on the assumption that these differ- 
ences are constant, and that no transitional forms between 
the English and the Californian crayfishes are to be 
met with, the individuals which present the characteristic 
peculiarities of the latter are said to form a distinct species, 
Astacus nigrescens ; and the definition of that species is, 
like that of the English species, a morphological abstrac- 
tion, embodying an account of the plan of that species, 
so far as it is distinct from that of other crayfishes. 
We shall see by and by that there are sundry other 
kinds of crayfishes, which differ no more from the English 
or the Californian kinds, than these do from one an- 
other ; and, therefore, they are all grouped as species of 
the one genus, Astacus. 
Tf, leaving California, we cross the Rocky Mountains 
and enter the eastern States of the North American 
Union, many sorts of crayfishes, which would at once be 
recognised as such by any. English visitor, will be found 
to be abundant. But on careful examination it will be 
discovered that all of these differ, both from the English 
crayfish, and from Astacus nigrescens, to a much greater 
