Description of a Species of Caligus. 237 
curved pinnula, which extends over the median line. The sec- 
ond joint is very large and increases in size from its base to its 
apex. ‘There isa short seta near its posterior margin, and another 
near its apex. On its posterior edge there is a broad membrane, 
exceedingly thin and transparent, and finely striated like the mar- 
gin of the shell. 
- The appendage to this joint arises from the inner part of its ex- 
tremity, and curves backward and inward. It is provided with 
seven long pinnule, which in general, extend with a curve to the 
median line of the body ; the first joint has one of these pinnule, 
the second two, and the third four. These joints are mostly very 
flat. The first joint is short and very similar in form to the basal 
joint of the leg. It is furnished exteriorly with a broad plate, 
which is ciliated at its apex. The second joint of this appendage, 
is smallest at its base, and increases with a curve to its apex, which 
is rounded. Its outer margin is ciliated. A shallow concavity 
receives the apical joint, which is small and nearly semicircular. 
The remaining joints of this leg, are also furnished with seven 
long natatory pinnule, of which one appertains to the third joint of 
the leg, one to the fourth, and five to the fifth. The third joint is 
broad and oblong, and is ciliated on its inner margin. A large 
stout spine, with a thin corneous expansion on two opposite sides, 
is articulated with the apex of this joint. The fourth joint is short- 
er than broad, and has a small spine at its apex. The terminal 
has nearly equal length and breadth, and is obliquely truncated at 
its extremity. There are two short spines at its apex. The pin- 
nula terminating this joint, is provided with ciliz only on its inner 
margin. ‘The other margin is furnished with a thin membrane, 
which extends from the apex, to an enlargement in the seta near 
its base. 
The sternum uniting the legs of this pair, is quite large; its 
breadth is one third its length. The posterior margin is some- 
what fleshy and thin, and provided with a delicate membrane, 
whose breadth is nearly as great as that of the sternum. The 
anterior articulating margin of this sternum is firm and osseous. 
The adjacent teguments with which it is articulated are similarly 
ossified, and supported both before and behind by two strong os- 
seous processes, situated in the teguments. The anterior processes 
are short, and terminate in a curve between the two sternums. 
The posterior are nearly three times the length of the sternum ; 
