100 
HISTORY OF BRITISH CRUSTACEA. 
Fam. ASTACIJDAE, Leach , 
Outer antennse with a ^movable plate at the base. Body 
elongated and slightly compressed ; carapace well deve- 
loped, with a moderate-sized beak covering the base of the 
eye-stalks. First pair of legs very large and two-clawed ; 
second and third pairs slender and two-clawed ; ' fourth i 
and fifth pairs one-clawed. Abdomen nearly of the same e 
breadth throughout ; side of each segment extended like a i 
plate, so as to cover more or less completely the base of the ; 
false legs. Tail very large, and broad outer plate on each i 
side with a transverse joint. 
Gen. 35. POTAMOBIUS, Leach, 
Beak depressed and very wide at the base, and armed on r 
each side with a tooth. Last thoracic ring movable. 
A genus confined to fresh-water. It is found in rivers s 
and brooks. The species are very voracious. 
Potamobius fluviatilis.^ The Liver Cray-fish, (Plate 
VII. fig. 4.) — Beak of the length of the peduncle of outer rj 
antennse, each side armed with a tooth situated about one- 
third from the end, and furnished with a slight elevation 
* Cancer Astacns, L Astacus fluviatilis, Fabr Potamobius fluviatiJis, Leach. 
