_Mouth._--Mandibles, with the inferior point produced into a single pectinated tooth, rarely into two pectinated teeth; on one side of one specimen, there were only four instead of five teeth. Palpi very narrow. Maxillæ highly variable; they may be described as formed of five steps, of which the two lower ones are generally united into a single one, divided by a mere trace of a notch; or with the three lower steps blended into an irregular, projecting surface, and with even the fourth step indistinct. I have seen these two extreme forms on opposite sides of the mouth of the same individual,--on one side the maxillæ being regularly step-form, on the other the whole inferior part forming an almost straight edge, standing high up above the first notch or step which bears the two upper great spines. _Cirri._--First pair rather far removed from the second pair, with the longer ramus about three-fourths of the length of shorter ramus of second cirrus; spine-bearing surfaces, hardly at all protuberant; lateral marginal spines on the posterior cirri rather long; caudal appendages smooth, rounded, extremely minute: penis very spinose. _Size._--Capitulum in the largest specimen, six-tenths of an inch long; only a few arrive at this size. _Colours_, after having been kept in spirits,--sack and cirri, especially first cirrus, clouded with pale purple; peduncle brownish; valves appear blueish in specimens not long preserved, but in specimens kept longer they become perfectly and delicately white. _General Remarks._--Under the head of _L. anserifera_, I have made some remarks on the diagnostic characters of this species. In the thinness of the valves,--form of the carina, with the rim connecting the prongs being not, or scarcely, reflexed,--and in the shortness and narrowness of the peduncle, there is some approach to _L. australis_, and thence to _L. fascicularis_. In the form of the maxillæ,--in one specimen having the mandible on one side bearing only four teeth,--and in the frequent absence of filamentary appendages, there is some approach to the genus _Pæcilasma_; but there is no such approach in the characters derived from the capitulum. We have seen that, as in so many other species of this genus, most of the parts are variable, and this is the case to a most unusual extent in the form of the maxillæ. Dr. Leach has attached eight specific names to the specimens preserved in the British Museum. 5. LEPAS AUSTRALIS. Pl. I, fig. 5. _L. valvis glabris, tenuibus, fragilibus; scutorum dentibus umbonalibus utrinque internis; carinæ parte superiore latâ, planâ, suprâ furcam valdè constrictâ; furcæ cruribus latis, planis, tenuibus, acuminatis, intermedio margine non relexo._ Valves smooth, thin, brittle; scuta with internal umbonal teeth on both sides. Carina with the upper part broad, flat; much constricted above the fork, which has wide, flat, thin, pointed prongs, with the intermediate rim not reflexed. Filaments, two on each side.