Perna ephippium. P. obliqua. P. isognomon. P. avicularis. P. femoralis. P. canina. P. marsupiom. P. sulcata. P. vulsella. P. nucleus. _P. femoralis._ The femoral Perna. Pl. 14, fig. 1. Species elongated, and with appendages like ears. _P. vulsella._ The tweezer Perna. Species elongated, without earlike appendages, or having very small ones. _P. ephippium._ The saddle Perna. Species round, compressed, very pearly in the interior; very slightly, if at all, auricled; margin acute; exterior purplish brown. 3. Malleus. The Hammer. Six species. A shell of a singular form, resembling a pickaxe, found only in the Indian and Australian Seas; there are two species, the white and the black, both of which, when in fine preservation, are highly esteemed, but the white is more rare and valuable. Shell sub-nacreous, irregular, rugged, sub-equivalve, inequilateral, generally auricled before and prolonged behind, so as to be in form like a hammer; summit entirely anterior; between them and the inferior auricle, an oblique cut or slope for the passage of the byssus; hinge linear, elongated, without teeth; ligament simple, triangular, inserted in a conical oblique cavity, partly exterior; one rather large sub-central muscular impression. Malleus albus. M. vulgaris. M. normalis. M. vulsellatus. M. anatinus. M. decurtatus. _M. vulgaris._ The common Malleus. Pl. 14, fig. 4. Species with two earlike appendages; trilobate, colour blackish brown. _M. albus._ The white Malleus. Species trilobate; base of the lateral lobe prolonged, without a sinus; the base and the pit of the ligament not distinct; colour white, with transverse undulations. _M. vulsellatus._ The tweezer Malleus. Species slightly auricled. _M. normalis._ The square Malleus. Species without earlike appendages. 4. Avicula. Thirteen species. Remarkable for the form of its shell, which resembles, when partially expanded, a bird flying. Shell foliaceous or not, thin and very fragile, always pearly, sub-equivalve; form sub-regular, but rather variable; summit anterior; valves oblique, the left one with a little notch, through which the byssus passes; sometimes unequally and obliquely auricled; hinge linear, toothless, or with two small rudimentary teeth; ligament more or less exterior, placed in a narrow groove, sometimes enlarged towards the summit; one very large posterior muscular impression; and one very small anterior.