Has the form of a cylindrical tube, slightly curved, and open at both ends; one extremity closed by four trapezoidal valves attached to the inner margin, the other end closed by a membrane. The annular ribs which separate the compartments show its progressive growth. Found buried so deeply in the fat of marine animals, particularly whales, that only the operculum and the upper part of the tube are visible. Shell rather elevated, sub-cylindrical, the partitions rather small and indented; the spaces or compartments almost quadrilateral; the inferior much more narrow than the others; the apertures equal and circular; the membrane which closes the superior forming a tube between the four almost equal valves of the operculum. _T. balænarum._ The Whale Tubicinella. Tubular, with transverse ribs, and a ring-shaped margin; operculum bottle-shaped. 2. Coronula. Three species. Found imbedded in the skin of whales and other marine animals, though not generally at so great a depth as the Tubicinella. Shell in form a little variable and without trace of support; the coronary part formed of six pieces, as in those properly called _Balanus_, but more regularly disposed in a manner to imitate a kind of crown or tube; spaces alternately hollowed and saliant; operculum not articulated, composed of two pairs of small, level, delicate valves, joined at the aperture of the tube by a considerable membranous part, leaving a passage for the cirrhous appendages of the animal. Coronula testudinaria. C. balænarum. C. diadema. C. quinquevalvis. _C. testudinaria._ The Tortoise Coronula. Very depressed, circular, as if radiated by the disposition of marked spaces, striated transversely, forming six rays diverging from the centre to the circumference; aperture oval and hexagonal. _C. balænarum._ The Whale Coronula. A little more elevated; the spaces prominent, equal between them, much larger than the excavated; the aperture subcircular; the operculum of four valves, almost equal, occupying but a small space of the membranous part which forms between them a sort of tube. _C. diadema._ The Crown Coronula. More elevated, sub-hexagonal; the spaces almost equal, the hollow parts larger than the saliant; the superior aperture very large and hexagonal; the inferior much smaller, of the same form, and communicating in a round excavation by radiated plates; the operculum bivalve. _C. quinquevalvis._ The five-valved Coronula. Pl. 4, fig. 3. Species irregular, having only five valves, of a purple hue. 3. Balanus. The Acorn Shell. Twenty-eight species.