_Cirri._--Prosoma very little developed. First cirrus very far removed from the second. The three posterior cirri are straight and long; the segments are elongated and bear four or five pairs of very long spines, with a single minute intermediate spine between each pair; dorsal tufts, with long spines. First cirrus, rami unequal by two or three segments, and thickly covered with spines; the first cirrus is short compared to the second, owing to the length of the pedicel of the latter, though the longer ramus of the first, nearly equals the shorter ramus of the second pair. Second cirrus, with its anterior ramus shorter by two or three segments than the posterior ramus, and thicker than it, with the segments covered like brushes with bristles; posterior ramus, and both rami of the third cirrus, a little more thickly clothed with bristles than are the three posterior cirri. _Caudal Appendages_, minute, broadly oval, with six or seven long bristles on their summits. _Genus_--CONCHODERMA. Plate III. CONCHODERMA. _Olfers._ Magaz. der Gesellsch. Natuforsch. Freunde zu Berlin, Drittes Quartel, 1814.[34] LEPAS. _Linnæus._ Systema Naturæ, 1767. BRANTA. _Oken._ Lehrbuch der Naturgeschichte, Th. 2, p. 362, 1815. MALACOTTA et SENOCLITA. _Schumacher._ Essai d’un Nouveau Syst. des Habitations des Vers., 1817. OTION et CINERAS. _Leach._ Journal de Phys., vol. lxxxv, p. 67, July, 1817. GYMNOLEPAS. _De Blainville._ Dict. des Sci. Nat., Art. Mollusca, 1824. PAMINA. _J. E. Gray._ Annals of Philosophy, vol. x, (Second Series,) August, 1825.[35] [34] The general title to the volume, containing four Quarterly parts, is dated 1818; but as in the ‘Journal de Physique,’ for July, 1817, the editor refers to Conchoderma, the Quarterly Part containing this genus must have appeared before 1818: Lamarck gives the year 1814 as the date of the paper in question, and I have accordingly followed him. From a similar reference by the editor, it appears that Schumacher’s volume appeared before the number of the ‘Journal de Physique’ containing Leach’s Paper. [35] Under these nine generic names, the two common species of Conchoderma have received thirty-three different specific denominations, caused partly by changes of nomenclature, and partly from varieties having ranked as species. _Valvæ 2 ad 5, minutæ, inter se remotæ: scuta bi-aut tri-lobata, umbonibus in medio marginis occludentis positis: carina arcuata, terminis utrinque pæne similibus._ Valves 2 to 5, minute, remote from each other: scuta with two or three lobes, with their umbones in the middle of the occludent margin: carina arched, upper and lower ends nearly alike. Filaments seated beneath the basal articulations of the first pair of cirri, and on the pedicels of four or five anterior pairs; mandibles, with five teeth, finely pectinated; maxillæ step-formed; caudal appendages, none.