176 Prof. F. M'Coy on the Classification of 



and smooth, or with few very minute teeth ; beneath the orbits 

 the cheeks are prolonged forwards about half the length of the 

 rostrum, and usually strongly keeled and spinose, forming a 

 semicylindrical sheath over the base of the strong triangular 

 scale of the origin of the outer antennae, which reaches 

 as far as the rostrum ; nuchal furrow strongly marked across 

 the middle of the back, but not reaching the marginal third 

 of each side; cheeks* impressed by a deep X-shaped sulcus, 

 one portion of which extends upwards nearly parallel with the 

 nuchal furrow, the longer lower branch curves forward under 

 the projecting part of the cheeks, and the shorter branch 

 curves backwards under the end of the nuchal furrow ; abdo- 

 men subcylindrical, smooth or slightly punctured, the second 

 joint having broad, dilated quadrate ends, the third, fourth, 

 and fifth terminating in triangular or broadly falcate extre- 

 mities, the sixth having articulated to each end the two outer 

 pairs of large trigonal tail-fins, the outer one on each side divided 

 by a transverse suture rather less than one-third from the ex- 

 tremity ; seventh joint (or middle flap of the tail) oblong, sides 

 denticulated, extremity narrower than the base, and bearing a 

 small spine at each corner ; first pair of legs very long and 

 thick, unequal, the larger claw with large blunt teeth, the 

 more slender one with more numerous and equal smaller sharp 

 teeth ; the other legs slender. 



In the general characters, so far as I have been able to ascer- 

 tain them, these crustaceans coincide with the living genus Ho- 

 marus, but are constantly distinguished by the sheath-like pro- 

 longation of the strongly ridged and spinose cheeks, the nearly 

 smooth- sided rostrum, and the short distance which the nuchal 

 furrow extends down the sides, as well as the separate X-shaped 

 cheek-furrow on each side, and the size of the antennary scale. 

 There are several species common in the British eocene tertiary 

 and cretaceous rocks, only one of which has yet been noticed, 

 viz. the Astacus longimanus of G. Sowerby (Zoological Journal, 

 vol. ii. tab. 17) from the greensand of Lyme .Regis, which I find 

 to belong to the present genus, and which should have the name 

 Hoploparia longimana (Sow. sp.). 



Hoploparia prismatica (M'Coy). 



Sp. Char. Carapace (excluding the rostrum) 1^ inch long, width 

 10 lines, subcylindrical behind, but having the section of 

 a five- sided prism towards the front from the strong pro- 

 jection of the large, acutely angular cheek-ridges, which bear 



* Or sides of the carapace immediately in front of each end of the nuchal 

 furrow. 



