14 ASTACID^. 



careful and right choice of the siDccific characters seems to depend 

 largely upon the taste of the describer and upon the richness of the 

 materials at his command. 



Accidental Variations. — In the first place, all accidental variations are 

 to be excluded. Crustacea are known very easily to reproduce parts 

 damaged or accidentally lost ; but these regenerated parts do not often 

 reach the size or form of the old j^ortions. Of course I do not here 

 speak of variations, which are very easy to be recognized, as when one or 

 more limbs are reproduced on one side. But I have seen specimens 

 with the same limbs renewed on both sides, and such specimens, 

 especially if one do not have before him more examples, are some- 

 times very difficult to recognize, and they often occasion errors (c£ A. 

 Ganit)clii). The most prominent parts of the Crustacea are easily dam- 

 aged and reproduced, as the claws, the antennae and their basal lamina, 

 the rostrum, and the caudal lamellae. But sometimes the more pro- 

 tected parts are found affected in the same manner, as the first pair of 

 abdominal legs, and even the epistoma. 



Differences of Age. — The differences of age are of vast importance, and 

 have a great influence on the right understanding of any species. The 

 very young and newly hatched animals are, without douljt, recognizable 

 by anybody. Afterward, as stated, there is more difficulty. The rich 

 materials of the Museum allow the following remarlvs : — 



Though generally the body has a constant form, I agree with Mr. 

 Gerstfeldt, that even here there is no mathematical identity. The oval 

 shape of the body, its greater or less convexity, its compression or de- 

 pression, the abrupt or gradual curving of its sides, are somewhat 

 variable. These variations, it is true, do not exceed certain limits ; 

 but they are here more troublesome, as in the great number of species 

 they seem sufficient to obliterate the specific characters. I say ex- 

 pressly '' seem sufficient," as the greatest difficulty consists in expressing 

 these differences clearly and definitely by words, while the worker, who 

 sees the species before him, separates them more easily. 



Differences in the Shape of the Head. — A more essential difference in 

 the shape of the head and the great claws is to be found in specimens 

 of different ages. Here these difierences are more striking and more 

 troublesome than the relations of the different parts. Their length and 

 breadth, their thickness, compared with that of the bod}^ and its limbs, 

 undergo marked fluctuations, and these fluctuations are fiir from being 

 of the same kind in all the groups of our fresh-water Crustacea. In the 

 several species of the group, of which Cambarns acidtis is the type, the 

 young animals always have a three-toothed rostrum, though in the older 

 individuals the lateral teeth are often nearly or entirely obliterated. 

 In the group of Cand>arus Bartonii, on the contrary, the younger ani- 

 mals do not difler in the shape of the rostrum from the older, although 



