(Verrill, 1874), a subtidal syllid polychaete associated with sponges, is described from the mouth of the St. Croix River. Distinguishing features are the dentition of the blades of the compound setae and the multicuspid pharyngeal teeth. Biological and morphological data on the common interstitial syllid, Sphaerosyllis brevifrons
Webster and Benedict 1884, are presented. This species occurs intertidally in coarse sand in the northern Gulf of Maine and is subtidal in the southern part of the Gulf.