^M. pilosa.^ An incrusting variety, composed of a single layer of cells spreading irregularly over the surfaces of stones, shells, and seaweeds, sometimes completely covering the fronds of /Chondrus crispus/, /Phyllophora/, /Rhodymenia/, and other algæ. The cells are membranaceous, with a calcareous rim, and have one long hair and several small ones surrounding the large roundish apertures. It is found in abundance on the shores from Long Island Sound to the Arctic Ocean. [Illustration: 1, /Membranipora pilosa/; a few of the cells seen from above, magnified. 2, /Membranipora pilosa/; a single cell, seen in profile.] ^M. lineata.^ Cells oblong, crowded, closely adherent; slender spines on edge of aperture, which bend over and meet across it; incrusts rocks and shells in broad, thin, radiating patches; cells [pg197] much smaller and narrower than those of /M. pilosa/; easily distinguished from that species by its manner of spreading and the absence of the one long hair. Common from New Jersey northward. ^M. tenuis.^ Common on pebbles, often covering their whole surface with a lace-like incrustation of very small oblong cells having a three-lobed aperture. Found in Long Island Sound and north to Cape Cod. GENUS ^Escharella^ ^E. variabilis.^ This species forms calcareous incrustations on shells or pebbles. The layers are thin, but eventually overlap one another, and the incrustation sometimes becomes an inch thick and resembles coral. The color of the living animal is dull red. The species ranges from Cape Cod to South Carolina, and is abundant in Long Island Sound. GENUS ^Mollia^ ^M. hyalina.^ Cells subcylindrical, irregular, transparent, smooth, and more or less oblique. The species forms small circular disks on algæ in tide-pools. GENUS ^Cellepora^ ^C. scabra.^ It forms branching, coral-like masses on slender red algæ. ^C. ramulosa.^ Branches cylindrical, rough, dividing in a forking manner, spreading, two to three inches high, calcareous; cells urn-shaped, irregularly arranged, apertures contracted, long spine on the outer edge. It is found in deep water attached to shells, and in tide-pools, where it grows chiefly on /Sertularia/ and other hydroids and on slender red algæ. It ranges from Long Island Sound to Greenland. (Plate LI.) ^C. pumicosa.^ Cells urn-shaped, irregularly crowded together, forming a conglomeration of porous, brittle masses, not exceeding an inch in breadth, usually round when small, oblong and knobbed when large; spine on outer margin of aperture. It incrusts stones and the stems of /Fucus/. (Plate LI.) SUBORDER ~CTENOSTOMATA~ GENUS ^Alcyonidium^