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. ( Cancer astacus, 
Linn.) 
In the fourth section, that of CARIDES, the middle an- 
tennae 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- 
tenna; ; the mandibles of the majority are narrowed, and 
arched at their extremity ; one of the first two pair of feet is 
olten folded on itself, or doubled ; the segments of the tail 
are dilated or widened laterally ; the external leaflet of the 
terminal fm 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 foliaccous. 
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 
