244 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH. 
so like our own, that it is necessary to compare the two in 
every point mentioned in the foregoing description in 
order to estimate the value of the differences which they 
present. Thus, to take one of the kinds of crayfishes found 
in California, which has been called Astacus nigrescens ; 
the general structure of the animal may be described in 
precisely the same terms as those used for the English 
crayfish. Even the branchie present no important 
difference, except that the rudimentary pleurobranchie 
are rather more conspicuous; and that there is a third 
small one, in front of the two which correspond with those 
possessed by the English crayfish. 
The Californian crayfish is larger and somewhat diffe- 
rently coloured, the undersides of the forceps particularly 
presenting a red hue. The limbs, and especially the 
forceps of the males, are relatively longer; the chele of 
the forceps have more slender proportions; the areola is 
narrower relatively to the transverse diameter of the 
carapace (fig. 61, C). More definite distinctions are to be 
found in the rostrum, which is almost parallel-sided for 
two-thirds of its. length, then gives off two strong lateral 
spines and suddenly narrows to its apex. Behind these 
spines, the raised lateral edges of the rostrum present five 
or six other spines which diminish in size from before 
backwards. The postorbital spine is very prominent, 
but the ridge is represented, in front, by the base of this 
spine, which is slightly grooved; and behind, by a distinct 
spine which is not so strong as the postorbital spine. 
