58 ASTACIDJE. 



is mostly truncated before in front. The basal joint of the inner an- 

 tenna' has a spine beneath, in the middle, or nearer the tip. 



As yet burrowing habits have not been observed in the specie- be- 

 longing to this group. But perhaps C. obscurus is identical with Astacus 

 fossor Rafinesque, which burrows in meadows and milldams. 



The species contained in this group are divided into four sections, or 

 perhaps they more fitly form one natural group and three abnormal or 

 exaggerated species, which ma}' be described in the following terms: — 



1. The rostrum is very long and acute: the lamina of the antenna' 

 elongated, dilated near the base ; the hands are narrow and elongated; 

 the first abdominal legs somewhat flattened at the tip. (C. lancifer.) 



2. The rostrum is more subquadrangular ; the lamina of the an- 

 tenna:- small, short, dilated in the middle: the hands are shorter and 

 broader; the first abdominal legs acute at the tip. | C. affords and allied 

 species.) 



3. The rostrum is short, conical, toothless ; the other characters are 

 as in the foregoing groups. ( C. immutris.) 



•4. Characters as in the groups of C. affirm, but the first abdominal 

 legs recurved at their extremity, the tip of which is rounded (as in C. 

 Bartonii). 



Synopsis of the Species. 



1st Section. 



1. C. lancifer. 



2d Section. 



2. .1. The margins of the excavated rostrum thickened. 



a. The margins straight, front border of the cephalothorax angulated : C. affinis. 



b. The margins concave, front border of the cephalothorax not angulated : C. juvenilis and 



C. placidus. 

 B. The margins of the flattened rostrum not thickened, front border of the cephalothorax not 

 angulated. 



a. The rostrum carinated at the tip : C. propinquus. 



b. The rostrum not carinated : C. virttis, C. rusticus, C. obscurus. 



3d Section. 



3. C. immunis. 



4th Section". 



4. C. extraneus. 



I have already mentioned that the species more closely related to C. 

 affinis are separated into two sections, one with the front border of the 

 cephalothorax angulated. — C. nffinis. ('. lancifer, 0. immunis, C. extra 

 the second with the front border straight or slightly notched. — O.juve- 

 . c. placidus, C. propinquus, C. viriUs, 0. rusticus, 0. obscurus. The lat- 

 ter section is very natural, and perhaps it would be preferable to follow 

 this arrangement in the separation of the species described. 



