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this character according with some difference in its habits ; 
and the maximum of specialisation in this direction is 
reached in Polybius henslowz, in which all the limbs, other 
than the chelipeds, have their terminal joints flattened and 
fringed with hairs. Correlated with this adaptation of the 
limbs in Polybius is a lightening of the whole structure, the 
shelly matter being reduced to a minimum thickness con- 
sistent with strength, and remarkably thin when compared 
with some other swimming crabs. It must not be supposed 
that these crabs are continually swimming; such a habit 
would expose them to too much danger from many fishes 
that already contrive to subsist largely on a crustacean diet, 
in spite of the many methods of concealment resorted to by 
crabs of all kinds. The swimming crabs are also diggers, 
rapidly going backwards into the sandy bottom or beneath 
stones to be secure from observation, yet keeping the eyes 
and antennez free from cover. Polybius has the terminal 
joints of the 5th pair of legs of an oval form, three quarters 
of an inch long and half as broad; the penultimate joint is 
also greatly flattened, so that the two constitute a very 
efficient paddle more than an inch long. But the 2nd, 3rd, 
and 4th legs, though considerably flattened, have their 
terminal joints shaped more like the claws of a lion, and 
rounded behind. This form admirably combines the swim- 
ming and digging powers in one instrument. Polybius is 
only found in or on deep water, and it has the reputation 
among fishermen of chasing fishes so swift as mackerel and 
pollack. It rests almost buried in the sand at the bottom, 
and apparently mounts rapidly towards the upper waters 
when it observes its prey. Portunus depurator has hinder 
limbs very similar to those of Polybius, both in shape and 
size, but the 2nd, 3rd, and 4th pairs are quite devoid of the 
swimming character, and agree with those of Carcinus menas, 
whose 2nd to 5th pairs are merely strong ‘‘ picks” for 
burrowing. Portunus puber, the largest of our native swim- 
mers, has the 5th pair of legs highly developed for swimming ; 
but as this crab spends much of its time under stones and 
gravel, the terminal joint ends in a sharp point, and is 
strengthened by a marginal and a central longitudinal band of 
thicker material. The entire build of this species is heavier 
and more solid than that of the others. The exhibitor 
pointed out corresponding differences in the other species 
exhibited, which he believed were due to slight differences 
of habit and habitat. The remarkable division of the antero- 
lateral margin into five teeth or spines on each side is a 
