Shell. External, orbicular, subirregular, nearly flat, slightly convex above, white, with apex near the middle; margin acute, internal surface rather concave; having a callous disk, coloured, depressed in the centre, surrounded by a smooth border. Inhabits the Indian Ocean. Two species.
- Umbrella Indica.
- U. Mediterranea.
6. Genus Pleurobranchus. Pl. [IX].
Animal. Body creeping, fleshy; mantle and foot expanded; branchiæ placed on the right side; cloak enveloping the shell; neck short, contracted in some species, with an emarginate front, exhibiting the commencement of the inferior tentacula, the upper ones tubular and cloven; gills at the edge of the dorsal plait, mouth provided with a short, retractile proboscis.
Shell. Depressed, oval, oblong, concentrically wrinkled, almost entirely open, rounded and convoluted; the vertex with a single turn. Inhabits the coast of Devonshire in England. Two species.
- Pleurobranchus Peronii.
- P. Laqueare.
FAMILY III.
Calyptracea. Seven genera.
1. Genus Parmophorus. Pl. [IX].
Animal. Body creeping, thick, oblong-ovate, broad behind, obtuse at the extremities; border of the mantle cleft before, and suspended vertically around; head distinct, and slit below; two conical contracted tentacula, at the base of which are placed the eyes, which are somewhat pedunculated; mouth below, funnel-shaped, oblique, truncated, and concealed; branchial cavity opening anteriorly behind the head by a transverse fissure.
Shell. Oblong, very depressed, slightly convex above, obtuse at extremities, anteriorly channeled by a slight sinus, and having towards the posterior part a small pointed apex, inclined backwards; the lower surface slightly concave. Inhabits the Australian seas. Four species. One fossil.