100 HISTORY OF BRITISH CRUSTACEA. 



Fam. AST ACID AE, Leach. 



Outer antennae 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 

 and fifth pairs one-clawed. Abdomen nearly of the same 

 breadth throughout ; side of each segment extended like a 

 plate, so as to cover more or less completely the base of the 

 false legs. Tail very large, and broad outer plate on each 

 side with a transverse joint. 



Gen. 35. POTAMOBIUS, Leach. 



Beak depressed and very wide at the base, and armed on 

 each side with a tooth. Last thoracic ring movable. 



A genus confined to fresh- water. It is found in rivers 

 and brooks. The species are very voracious. 



Potamoeius fluviatilis.* The River Cray-fish. (Plate 

 VII, fig. 4.) — Beak of the length of the peduncle of outer 

 antennae, each side armed with a tooth situated about one- 

 third from the end, and furnished with a slight elevation 



* Cancer Astacus, L.; Astacus fiuviatilis, Tahr.; Potaruobius fluviatiJis, Leach. 



