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.
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.
The great confusion that existed in this genus, as classed by Linnæus, has been elucidated by the division and classification of Lamarck. This genus derived its name from the resemblance which some of the smaller species have to a bubble of water. Its shells are found in almost all parts of the world.
Shell external, oval, involuted; aperture very large, open the whole length of the shell, and generally wider at the base; outer edge sharp and smooth; summit umbilicated.
Bulla lignaria. B. ampulla. B. striata. B. naucum. B. physis. B. fasciata. B. aplustre. B. hydatis. B. cornea. B. fragilis. B. solida.
_B. aplustre._ The streamer-like Bulla. Pl. 17, fig. 7.
Species completely involute; the spire very distinct, visible, but not projecting, with a kind of thickening at the anterior part of the columellar edge.
Species sub-involute, no visible spire either within or without, but narrowed towards the top when it is slightly umbilicated; yellowish brown colour, with transverse pale striæ.
Species more solid, thicker, almost entirely involute; whorls of the spire slightly visible in an umbilicus projecting interiorly from the summit.
Species thin; the whorls of the spire visible externally, but without projection, and with a suture as if caniculated without thickening at the anterior part of the columellar edge.
Species very thin, rather involute; the whorls of the spire distinct within as without; the suture deep, angular, and cleft in a greater or less part of its length.
This genus may almost be said to have no testaceous covering, as it appears more like the element of a shell.