g G. O. Sars. 



greater part of the dorsal face, as also laterally by the 

 great adductor muscle. Only the posterior, or abdominal 

 section is to a certain extent movable, admitting of being" 

 slightly extruded from the shell ventrally, whereas the 

 anterior section is quite immovable and deeply sunk within 

 the cavity of the shell. In front, this part is almost ver- 

 tically truncated, and accordingly no distinctly defined ce- 

 phalic section exists. At its upper corner, just above the 

 insertion of the superior antennæ, the simple eye is visible, 

 enveloped by a slight, rounded projection of the skin. 



The superior antennæ (fig. 5) are rather slender, each 

 consisting of a rather thick, muscular scape, and a slender, 

 flexible terminal part. The scape is composed of 2 imper- 

 fectly defined joints, the proximal of which is by far the 

 larger, and is strengthened by several chitinous stripes. At 

 about the middle of the convex upper edge, it bears a 

 slender, procurved seta, and 2 still more elongated setæ 

 are seen issuing- from the opposite edge, pointing in dif- 

 ferent directions. The distal joint of the scape gradually 

 tapers towards the end, and carries in front a single rather 

 short bristle. The terminal part is somewhat longer than 

 the scape, and is very movably articulated to its end, so as 

 to admit of rather extensive movements in a vertical plane. 

 It forms a slender stem composed of 5 sharply, defined ar- 

 ticulations, the 1st of which is by far the largest, and pro- 

 vided at the end, on each side, with a slender seta. The 

 4 succeeding articulations rapidly diminish in size, and each 

 carry both in front and behind 2 setæ, of which especially 

 the anterior ones are very long and finely plumous. All 

 these setæ form together a dense fascicle proceeding from the 

 tip of the antennæ, and during the movements of the ani- 

 mal they admit of being spread in a fan-like manner. 



