ASTACIDJE. 31 



near the extremity, sometimes, however, but very slightly developed, — 

 0. acutus and allied species. 



This character is indeed good, but sometimes not so striking as to 

 prevent a mistake, which even Mr. Grirard himself made, in placing C. 

 pcttucidus in his first group. There are some species in every group, 

 the exact place of which is uncertain. At first sight the C. spiadifcr 

 and C. versidus would be placed in the first group, and not in the third ; 

 C. penicillatus and C. Wiegmanni near C. Bartoni in the second group, and 

 not in the third ; C. immunis in the second, and not in the first • C. ad- 

 vena and C. Carolinus in the third, and not in the second, — to which in 

 reality they respectively belong. But generally, I repeat, this charac- 

 ter is good, if not prima vista, at least in connection with the other 

 characters. 



Nevertheless, in the rostrum is to be found another important char- 

 ter. In one group the rostrum never has lateral teeth at the tip, in all 

 others these lateral teeth are to be found, if not in the full-grown speci- 

 mens, yet always in the young. But the latter fact renders this mark 

 evidently less useful than the character taken from the hooked legs. 

 In the last case only the newly hatched specimens have no hooks and 

 are doubtful; but in the other instance only the newly hatched speci- 

 mens always have teeth, while the full grown are sometimes toothless. 

 C. acutus and the allied species always have in the young specimens 

 well-developed teeth, and the form of the rostrum is therefore altered 

 in such a manner as to place them prima vista in Mr. Girard's first 

 group, near C. affinis. C. penicillatus, though toothless when full grown, 

 has well-developed teeth when young. C. immunis, and in some degree 

 C. virilis, G. propincpius, and others, are in the same condition. 



According to the absence or presence of the lateral apical teeth of the 

 rostrum, Cambarus is divided hito two other groups, not coinciding with 

 those already mentioned : — 



I. Always toothless, — C. Bartoni and allied species. 



II. With teeth, at least in the young, — all the rest. 



Combining the two principal characters mentioned, — the number of 

 the hooked legs and the toothless or toothed rostrum, — we find three 

 well-defined groups : — 



I. Third and fourth legs hooked, rostrum toothed, — C. acutus and 

 allied species. 



II. Third legs hooked, rostrum toothed, — C. affinis and allied species. 



III. Third legs hooked, rostrum toothless, — C. Bartoni and allied 

 species. 



These groups coincide with those established by Mr. Girard, after re- 

 moving his erroneously placed species, viz. C. pcttucidus, C. Oreganus, C. 

 Gambeli. 



Some other characters serve to evince more or less clearly the natu- 

 ralness of these groups. 



