Differing from _armillaris_ (Müller), _alternata_ (Moore) and related forms in the form and relations of the prostomium and its appendages. The prostomium is broadest anteriorly, narrowing caudad and rounded forward a little at middle in front. A characteristic feature is that the three tentacles are in a transverse line along the anterior edge, the median being thus inserted far in advance of the posterior eyes. A median longitudinal furrow extending forward from caudal edge to base of median tentacle. The anterior eyes much larger than the posterior and farther apart, each somewhat transversely elliptic and located far forward at base of lateral tentacle on its ectal side. The median tentacle about two and a half times longer than the prostomium; in the type composed of twenty-one articles; only a little narrowing over the distal region. Lateral tentacles considerably shorter than the median. Inferior tentacular cirrus about equal in length to the median tentacle, the upper one much longer and consisting of about thirty-four articles. Neurocirri slender, subcylindric, somewhat conical distally or sometimes a little clavate, surpassing end of parapodium. Notocirri in anterior region alternating in length, the long ones surpassing the width of the body proper and consisting of about thirty-two articles while the short ones embrace only near eighteen. Notocirri becoming shorter and essentially uniform in the posterior region. Appendage of setae with subapical tooth larger and stouter, more obtuse, than in _alternata_, making a wider angle with the apical tooth, and always conspicuous; the serrations proximad of the tooth fine and rather long. The body is proportionately rather wide and is depressed or flattened, narrowing in the posterior region but retaining there the depressed form. Number of segments in the type, near one hundred and forty-five. General color yellowish; each somite of anterior region crossed transversely by two fine complete lines of reddish brown color. Width in anterior region, exclusive of parapodia, about 1.25 mm.; length near 20 mm. Type--M. C. Z. 2, 147. Taken at low tide. The type is a female turgid with eggs. It is remarkable in presenting at the same time a well-developed collateral bud from the ventral surface near the beginning of the posterior third. _Pionosyllis pigmentata_ sp. nov.