Species of which the shell is sometimes very large, white, transversely ovate or elongated; the anterior side longer than the posterior; broad ribs covered with vaulted scales; when of a pink or orange colour, greatly valued. 2. Hippopus. One species. Similar to the Tridacna, but distinguished from it by having its posterior slope closed with a dentated margin; its ribs are never arched or vaulted, and its anterior side is shorter than the posterior side. _H. maculatus._ The spotted Hippopus. Pl. 12, fig. 1. Shell transversely ovate, ventricose, with scaly ribs; lunule, heart-shaped, and oblique; margins deeply crenulated; reddish purple spots. FAMILY XVI. MYTILACEA. Three genera. 1. Mytilus. The Muscle. Thirty-five species. As arranged by Lamarck, now comprises only such shells as are regular, equivalve, and longitudinal; solid in substance, and attached to other bodies by a short thick byssus. In colour and appearance they greatly vary, some being smooth and beautifully variegated with delicate colours, or radiated with purple and white; some are coarsely ribbed or granulated, and have only one colour, as black, blue, yellow, brown, or green; all are covered with an epidermis, to which oftentimes the colour is confined. Shell of a close texture, elongated, more or less oval, sometimes sub-triangular, equivalve, summits anterior, more or less curved, a little sloping inferiorly; hinge generally without teeth, or with two very small rudimentary teeth; ligament dorsal, linear, sub-interior, inserted in a long and narrow furrow; two muscular impressions, of which the anterior is very small, with or without longitudinal grooves or furrows.