_C. extinctorium._ The extinguisher Calyptræa. Species in which the internal appendage is horn-shaped; colour brownish. _C. equestris._ The equestrian Calyptræa. Pl. 32, fig. 4. Species in which the internal appendage is like a horseshoe, open in front. _C. tectum-sinense._ The Chinese roof Calyptræa. Shell formed of separate, transverse, irregular round laminæ of uniform size, attached to each other by the summit on the exterior of each, presenting the appearance of a number of small flat Patellæ piled one on the other; colour yellowish, margin entire, very glossy within. 6. Crepidula. The Slipper Limpet. Six species. Very similar to the Navicella, but distinguished from it by not having an operculum. Shell irregular, form very variable, depressed or compressed; spire obliquely inclined to one side; margin entire; cavity large, divided by a horizontal partition, which gives it the form of a half-decked boat. Crepidula fornicata. C. porcellana. C. aculeata. C. unguiformis. C. dilatata. C. Peruviana. _C. porcellana._ The brown-spotted Crepidula. Shell thick, flat, summit not spiral. _C. aculeata._ The prickly Crepidula. Oval, brown, roughly striated, vertex recurved, interior blue or purple. _C. sub-spirata._ The sub-spiral Crepidula. Pl. 32, fig. 2. Species almost round, summit sub-spiral; colour yellowish white, with a bluish tinge towards the summit. 7. Ancylus. The Lake Limpet. Two species. This is a fresh-water shell, found in the lakes of Europe. Shell thin and brittle, obliquely conical; summit pointed and recurved; margins simple, base oval and smooth. Ancylus oblongus. Ancylus fluviatilis. _A. oblongus._ The oblong Ancylus. Aperture elongated, vertex turned to one side, striated concentrically; exterior pale yellowish colour, interior light blue. _A. fluviatilis._ The river Ancylus. Pl. 32, fig. 3. Simple, oval, almost symmetrical; summit pointed, compressed, very distinct; bent back a little to the right, but not marginal; the edges of the aperture entire and effuse. FAMILY IV. BULLACEA. Three genera. 1. Acera. Has no shell. One species. Acera Carnosa. 2. Bullæa. One species. Formerly classed with the Bulla, but separated from it on account of the shell being entirely covered by the animal, and never externally visible. Shell oval, thin, fragile, more or less involuted on one side, rendering the aperture more or less wide. _B. aperta._ The open Bullæa. Pl. 17, fig. 5. Shell interior and very incompletely involuted, without spire or columella; sub-orbicular, white, transparent, faintly striated, and slightly wrinkled; almost entirely open.