_Animal._ Body oblong, gelatinous, contractile; a head in front with two wings, and the posterior part enveloped in a shell, head distinct, projecting and round; two eyes; mouth in the form of a small beak; destitute of tentaculæ; two, opposite, membranaceous, pellucid, and cordated wings, placed at the base of the neck. _Shell._ Pyramidal, triangular, of a gelatinous or cartilaginous substance, very thin and transparent; aperture obliquely truncated. Inhabits the South American seas. Two species. Cleodora pyramidata. C. caudata. 4. Genus _Limacina_. Pl. IX. _Animal._ Body soft, oblong, two branchial fins situated at the base of the neck; posterior part of the body spiral, and enveloped in a shell. _Shell._ Thin, fragile, papyraceous, spiral, the whorls reunited in a planorbis form, and deeply and largely umbilicated on one side; aperture large and entire. Inhabits the North seas. One species. Limacina helicialis. 5. Genus _Cymbulia_. Pl. IX. _Animal._ Body oblong, gelatinous, pellucid, enclosing a shell; head sessile; two eyes, and two retractile tentacula; mouth with a retractile proboscis; two, opposite, oblong, ovate, branched wings, connate at their posterior base. _Shell._ Gelatinous, cartilaginous, very transparent, crystalline, oblong, in shape of a shoe, from which it has derived the name of the slipper, truncated at the summit; aperture lateral and anterior. Inhabits the Mediterranean. One species. Cymbulia Peronii. 6. Genus _Pneumodermon_. Pl. IX. A molluscous animal, without any testaceous covering, and much resembling the genus Clio. Inhabits the Indian seas. One species. Pneumodermon Peronii. FAMILY II. PHYLLIDIACEA. Six genera. 1. Genus _Phyllidia_. Pl. IX. A mollusca similar to the preceding genus, without a shell, but its back covered with a rough or coriaceous skin. Inhabits the Mediterranean. Three species. Phyllidia varicosa. P. pustulosa. P. ocellata. 2. Genus _Chitonellus_. Pl. IX. _Animal._ Body creeping, elongated; middle of the back provided its entire length with a detached, multivalve shell; the alternate pieces for the most part longitudinal; sides naked; branchiæ disposed around the body; foot cleft longitudinally by a deep furrow. _Shell._ Each valve with striæ radiating from its apex; the margins serrated; the base of the last valve obtuse. The testaceous plates of this genus are never joined like those of the Chiton, so that the animal can move in every direction. Upon the contraction of the animal after death, however, these valves become nearly united. Inhabits the seas of New Holland. Two species. Chitonellus striatus. C. larvæformis. 3. Genus _Chiton_. Pl. IX.