244 THE COMPARATIVE MOEPHOLOGT 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 brahchise present no important 

 difference, except that the rudimentary pleurobranchise 

 are rather more conspicuous ; and that there is a third 

 smaU 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 chelae 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. 



