122 
ASTACUS. 
which, from the delicacy of the fossil, and the 
liardness of the surroimding matrix, is exceedingly 
difficult and tedious, and can scarcely be accom- 
plished by an inexperienced hand. Some of the 
specimens in my cabinet exhibit the claws, others 
the thorax, and a few the abdomen and tail. From 
these detached parts a restored outline of the 
original was delineated, and its relation to the 
recent species by this means ascertained. I have 
named the first species 
Astacus Leachii^ in honour of Dr. Leach of the 
British Museum. The genuine and specific cha- 
racters are as follow : — 
Gen. Char. Antennas pedunculated, unequal, 
the exterior ones long and setaceous ; the inner 
pair divided at the extremities ; body elongated ; 
legs commonly ten ; tail foliaceous. 
Spec. Char. Thorax scabrous, convex, six- 
lobed, marginate ; head semicircular in front ; 
hands chelate, muricated, twice tlie length of the 
thorax ; pincers very long, armed with spines. 
The thorax is longitudinally oblong, convex, 
covered with small tubercles and papillas ; it is di- 
vided into six lobes by a rounded dorsal ridge, and 
two lateral sulci ; the margin is entire : the head 
appears to have been semicircular, or rounded in 
front, and is not distinct from the thorax. 
Tlie external antennae are long, filiform, and 
setaceous, and are placed on squamous peduncles ; 
the inner pair have not been discovered. The two 
chelate hand-claws are equal, and liave their sur- 
face miH'icated, or beset witli short erect spines, 
d'he pincers are very long, not muricated, but 
