Readily differentiated from _P. elongata_, which it resembles in its pale, translucent appearance, in having the distal appendage of setae more typical, being of a decidedly more elongate and erect form which also differs from that of _pigmentata_. From the latter differing conspicuously in appearance in lacking all dark pigment. Notocirri tapering distad, with apical region slender and pointed; long, exceeding the width of the body and consisting of up to forty-five articles. Differing from _pigmentata_ in the form of the neurocirri which are more uniform in diameter, subcylindric rather than fusiform; normally extending more or less beyond the tip of the parapodium. Prostomium short. Eyes reddish; those of first pair larger than the second; second eye on each side almost directly mesad of the first but only a little caudad of it. The median tentacle farther forward than in _pigmentata_, well in front of the eyes, its anterior edge being nearly in line with the caudal margins of the paired tentacles; composed of twenty-eight or more short articles. Paired tentacles much shorter and also more slender; composed of about twenty articles. Palpi fused at base as usual; narrower distally than in _pigmentata_. The types are incomplete caudally; but the body is evidently slender. One specimen 8 mm. long consists of forty-three segments; and a second, somewhat thicker one, of nearly the same length consists of thirty-seven. The width is near 1 mm. Type--M. C. Z. 2, 180. _Hesperalia_ gen. nov. Palpi thick, fused at base only to middle of length. Pharynx straight. Proboscis unarmed (?). Tentacles three, attenuated, more or less obviously jointed. Eyes two pairs; large. Tentacular cirri two pairs. Parapodia uniramous with setae all compound, or in the epitokous phase with long simple natatory setae in notopodia of middle region of body. Appendage of compound setate short, bidentate. Neurocirri present, thick, rounded. Notocirri on side of body above parapodia; filiform; more or less segmented. A large quadrate membrane or flap projecting from anterior edge of peristomium forward over caudal region of prostomium. _Genotype._--_H. californiensis_ sp. nov. _Hesperalia californiensis_ sp. nov.