A rather slender species with branchiæ of moderate length. Excepting the eyes with no pigmented markings. Radioles fifteen pairs. Eyes few, not present on all radioles; where present usually but a single one on each radiole, in one case two; the eyes deep purple, variable in side from moderate to small; situated at varying distance between base and middle of length of radioles. Free dorsal edge of branchial membrane with two short obtuse lobes overlapping in the middle line. The dorsal notch in the collar lobe on each side is mesad of the line of setigerous tubercles, wide open, rectangular or slightly obtuse; lobe mesad of notch small, anteriorly rounded, the mesal edge extending into the dorsal furrow; median ventral lobes separated by a narrow incision, short, the ectal edge passing out in an even concave curve to the anterior lateral margin. A characteristic feature of the species is the presence of ten setigerous thoracic somites. Dorsal setæ of the usual two types of which the upper are much fewer spatulate setæ in two series with distal expansion broad and wings asymmetrical, tip short. Total number of segments, seventy-eight.
Length without branchiæ, 31 mm.; with branchiæ, 36 mm.
The type of this species is a small individual which, as preserved, appears of a uniform dusky color throughout. Branchial radioles fourteen or fifteen pairs; in a single series, the membrane not being coiled. No eye spots. Collar with ventral lobes proportionately long and acute, the dorsal lobes small and approximate. Notopodial setæ of usual two types; few. Uncini with beak divergent, nearly horizontal, the "neck" short and the edge of body below bulging much as in _Paralaonome japonica_. Body furrowed along each side just above notopodia excepting anteriorly. Ventral plates sharply limited, elevated; all of abdominal plates bisected by the median longitudinal sulcus excepting the first one, which is entire. Total number of somites, fifty-six, of which eight are thoracic.
Length without branchiæ, 12 mm.; with branchiæ, near 15 mm.
Taken among tufted algæ, June 25, 1911 (C. F. Baker).
In this form the collar membrane is crossed by a series of longitudinal dark stripes, one in line with each radiole, as in _Potamilla colorata_, these narrowing caudad. Branchiæ sometimes mostly dark with light transverse bands. Anterior thoracic segments darkly pigmented both above and below, and also along both sides of tori, and most setigerous papillæ and tori of succeeding regions of body also surrounded in some degree with a pigmented area. Branchial membrane with free dorsal edges produced into two lobes on each side, the two of each pair overlapping, the posterior lobe rounded, the anterior angular with its caudal margin transverse and the other long and oblique. Radioles nineteen pairs, several of these at dorsomesal end of series much reduced. Eyes conspicuous but few, only one, or occasionally two, on a radiole and some radioles wholly lacking them. This species has only seven setigerous thoracic somites. Total number of segments, near ninety-four.