Animal. Body enclosed in a shell, winged before, two opposite wings, somewhat retractile, inserted at the sides of the mouth, head distinct, mouth terminal, placed at the junction of the fins; without eyes.
Shell. Symmetrical, very thin and transparent, valves unequal, flat above, convex below, open like a cleft anteriorly, summit truncated and tridentated posteriorly. Inhabits the Mediterranean. Two species.
- Hyalæa tridentata.
- H. cuspidata.
2. Genus Clio. Pl. [IX].
Animal. Body free, naked, more or less elongated, tapering to the rear, head very distinct; provided with six, long, conical, retractile tentaculæ, separated into two groups of three each; mouth altogether terminal and vertical, eyes sessile, rudiment of a foot under the neck.
This is a molluscous animal without any testaceous covering, but is here placed as leading to other genera which have shells. Inhabits the Indian seas. Two species.
- Clio borealis.
- C. Australis.
3. Genus Cleodora. Pl. [IX].
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.