As compared with _A. attenuata_ Treadwell, this is a smaller species differing in appearance in being brown of a decided greenish tinge, excepting on the prostomium and at the caudal end. The prostomium is less narrowed cephalad, being more broadly rounded across anterior end. Median eyes not exceeding the lateral in size. Maxillae V represented by simple small hooks. IV with five teeth of which the most ectal (upper) is long and slender, the two next much shorter and finer and the two innermost closer together. III with fine teeth similarly arranged and formed. Maxillae II nearly symmetrical; the left one with seven teeth of which the most anterior one is much largest, the right with an additional small tooth in front of (ectad of) the large one; neither of the plates extending caudad of the anterior end of the dental series of I. I with seven or eight well developed teeth; the carriers very long and slender, black throughout. In the paraphodia the posterior lobe is well developed, stout and conical, distally somewhat blunt or rounded, extended ectad or caudoectad and is always shorter than the setae. Setae all simple, limbate, in a single series of mostly six in the middle region of the body. Setae with the usual double or sigmoidal curve over the limbate part, the first bend or geniculation unusually strong, angular; tip becoming fine and hair-like. Body tapering caudad, pointed at the posterior end, ending in two blunt lobes. Number of segments in the type one hundred and ninety-one. Length, 46 mm.; width, exclusive of parapodia, 2 mm. Type--M. C. Z. 2, 155. Taken at the shore "under rocks." _Arabella mimetica_ sp. nov. Resembling the preceding species though smaller and more slender. Superficially differing obviously in the form of the prostomium which is much more narrowed distad and is neither depressed nor furrowed either dorsally or ventrally. Eyes smaller, obscure. Maxillae resembling those of the other species in general, but differing strongly in the second pair in which the right plate, instead of being symmetrical with the left one, is decidedly long and extends far proximad along the dental line of I and bears about fifteen teeth as against only six on the left one and eight on the corresponding plate in _lagunae_. Maxillae I on right side with nine teeth, on left apparently with seven. Maxillae III with teeth in arrangement as in _lagunae_ but only four in number and different in all being blunt and shorter. IV as in the other species but teeth four instead of five. The number of segments in the type is near one hundred and sixty-five. Length, 40 mm.; width, 1.1 mm. Type--M. C. Z. 2, 156. Taken among holdfasts of kelp. (C. F. Baker, June 30, 1911.) Also a small specimen taken August 2, 1917, by Prof. Hilton. _Biborin_ gen. nov.