ORDER DECAPODA. 
185 
the internal dilatation of the lower articulations of tlieir ex- 
ternal jaw-feet. Their body is very much flatted. 
Some are remarkable for their very large forceps, furnished 
with hairs, or very much ciliated. Such are 1st, Cancer 
platychiles, Penn, whose forceps are hairy only at the exter- 
nal edge, and thorax almost naked and rounded. 2nd, P. 
liirta, Lam., in which all the upper part of the forceps and 
thorax is hairy, and in which this part is almost oval, and 
narrowed in front. The others have the pincers without 
hair ; such is Cancer hexapus, Linn. 
The genus Mo node pis of M. Say appears to constitute 
the passage from porcellana to megalopus. It approaches 
the first in the relation of the two posterior feet and of the 
direction of the tail. But this tail should have but six seg- 
ments, and the eyes should be very large, as in the second. 
It would also appear that the lateral fins of the end of the tail 
resemble those of the last. 
The other Crustacea of the same division differ from the 
preceding by having their posterior feet similar in form, pro- 
portions, and uses, to the preceding, or equally ambulatory. 
They are again remote from them, by reason of their body, 
which is more thick and more raised, and of their lateral 
antennae, which are much shorter, also by reason of their 
claws being smaller, the bulk of the eyes and the lateral fins 
of the tail, which are composed but of a single lamina. This 
tail is extended, narrow, and simply curved underneath, to- 
wards its extremity. 
Megalopus, Leach — Macropa, Latr. 
We shall comprehend in our second division ( Astacini, Latr.) 
those which have five pair of false feet, the middle antennae 
straight, or almost straight, projecting, advanced, and termi- 
nated by two threads, as long or longer than their peduncle. 
