Shell equivalve, inequilateral, transverse, with small apices; recurved, but nearly erect; not auriculated; hinge very long, linear, crenulated through all its length; ligament external and marginal; two well-marked muscular impressions. _I. exotica._ The exotic Iridina. The only species of this genus answering to the above description. FAMILY XIV. CHAMACEA. Three genera. 1. Diceras. One fossil species. Only found in a fossil state; distinguished from the Chama by the hinge, which is dissimilar, formed by a large thick tooth, concave in the greater valve; summits very projecting; almost regular spiral contortions. _D. arietina._ The ram’s-horn Diceras. Shell irregular, inequivalve, inequilateral, somewhat heart-shaped, with divergent beaks. 2. Chama. The Clam or Gaper. Seventeen species. In this genus are now comprehended only such as have a thick oblique transverse tooth, resembling a lengthened callosity, generally crenulated or grooved, fitting into a corresponding cavity in the lower valve. The animals inhabiting these shells have the faculty of affixing themselves to other bodies by the lower valve. The Chama received its name from its gaping; it is found in most seas, particularly in the Southern; sometimes its colours are elegantly blended. Shell irregular, adherent, inequivalve, inequilateral; summits more or less twisted spirally, especially in the lower valve; some from left to right, others from right to left; hinge dissimilar, large, formed by one lamellous, arched, sub-crenulated, post-cardinal tooth, articulating into a furrow of the same form; exterior, post-apicial, slightly inserted; two large and rather distant muscular impressions. Chama Lazarus. C. damæcornis. C. gryphoides. C. crenulata. C. unicornis, C. arcinella. C. radians. C. cristella. C. florida. C. limbula. C. æruginosa. C. asperella. C. decussata. C. albida. C. ruderalis. C. croceata. C. Japonica. _C. Lazarus._ Lazarus’s Chama. Pl. 12, fig. 2. Species of which the summits twist from left to right; imbricated, dilated, waved foliations; striated obsoletely; exterior white, orange, red, or yellow; interior white. _C. arcinella._ The hedgehog Chama. Species of which the summits twist from right to left; the pink-coloured ones of this species are the most prized. 3. Etheria. Four species.