Shell very thin, transparent, symmetrical, convex below, flat above, valves unequal, form tricuspidated, cleft at the sides, open like a cleft anteriorly, and tridentated posteriorly; summit truncated. Hyalæa tridentata. Hyalæa cuspidata. _H. tridentata._ The three-toothed Hyalæa. Transparent, horn-coloured, globular; tridentated posteriorly; summit and two posterior sides open; finely striated transversely. 2. Clio. Has no Shell. Two species. Clio Borealis. Clio Australis. 3. Cleodora. Two species. Shell gelatinous, cartilaginous, transparent, in shape of a reversed pyramid or lanceolate truncated, only open at the summit. Cleodora pyramidata. Cleodora caudata. _C. pyramidata._ The pyramidal Cleodora. Like a pyramid, triangular, thin, transparent; aperture obliquely truncated. 4. Limacina. One species. Shell papyraceous, very fragile, planorbis form, sub-carinated, involuted rather obliquely, in such a manner as to be deeply and largely umbilicated on one side, and the spine slightly projecting and pointed on the other; aperture large, entire. _L. helicialis._ The Helix-like Limacina. Thin, fragile, spiral; the volutions united in a discoid form. 5. Cymbulia. The Slipper. One species. Shell or case cartilaginous, transparent, conical posteriorly where the animal adheres, and prolonged above like a long hollow semi-cylinder, under which the animal can take shelter. _C. Peronii._ Peron’s Cymbulia. Shell shaped like a shoe, somewhat gelatinous or cartilaginous, very transparent crystalline, oblong pointed at the vertex, truncated at the base. 6. Pneumodermon. Has no shell. One species. P. Peronii. FAMILY II. PHYLLIDIACEA. Four genera. 1. Phyllidia. Three species. The animals of this genus are covered with a coriaceous skin, but without a shell. Phyllidia varicosa. P. pustulosa. P. ocellata. 2. Chitonellus. Two species. Formerly classed with the Chiton; but as the testaceous plates of this genus are never joined, the two may be easily distinguished. Shell elongated, multivalve; alternate pieces generally longitudinal; sides naked. Chitonellus striatus. Chitonellus larvæformis. _C. striatus._ The striated Chitonellus. Striæ radiating from the apex of each valve; margin serrated; base of the last valve obtuse. _C. larvæformis._ The Caterpillar Chitonellus.