Description of a Species of Caligus. 237 



curved pinnula, which extends over the median hne. The sec- 

 ond joint is very large and increases in size from its base to its 

 apex. There is a 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 pinnulee, which in general, extend with a curve to the 

 median line of the body ; the first joint has one of these pinnulas, 

 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 pinnulse, 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 ciliee 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 ; 



