ORDER DECAPODA. 189 



pose, with the fresh-water species, the genus Astacus, pro- 

 perly so called, of the same naturalist. {Cancer gammarus, 

 Linn.) 



In the fresh-water species, which, in their antennae, eyes, 

 and the form of the claws, otherwise resemble the preceding, 

 the last segment of the tail, or the middle one of its terminal 

 fin, is cut transversely in two by a suture. {Cancel' astacus, 

 Linn.) 



In the fourth section, that of Carides, the middle an- 

 tenuEe are superior, or inserted above the lateral, the peduncle 

 of the latter is entirely covered by a large scale. 



Their body is arched, as it were humped, and of a con- 

 sistence less solid than that of the preceding Crustacea ; the 

 forehead is always prolonged in front into a point, and most 

 frequently in the manner of a bill, or pointed lamina, com- 

 pressed and denticulated on its two edges. The antennae 

 are always advanced ; the lateral ones are usually very long, 

 and in the form of a very attenuated thread ; the intermediate, 

 in a great number, terminate in three threads ; the eyes are 

 very much approximated ; the external jaw-feet, more nar- 

 row, and more elongated than usual, resemble palpi or an- 

 tennae ; the mandibles of the majority are narrowed, and 

 arched at their extremity ; one of the first two pair of feet is 

 often folded on itself, or doubled ; the segments of the tail 

 are dilated or widened laterally ; the external leaflet of the 

 terminal fin is always divided in two by a suture, a character 

 which is observed only in the last Crustacea of the preceding- 

 section. The odd piece of the middle, or the seventh and 

 last segment, is elongated, narrowed towards the end, and 

 presents, above, some ranges of small spines ; the false feet, 

 four pair in number, are elongated, and usually foliaceous. 



Some have the first three pair of feet in the form of a 

 didactylous claw, and their length augments progressively, so 

 that the third pair is the longest. Such are 



