In this genus the shell is thin, fragile, and colourless, of a broad rounded outline, with the aperture very wide, and a small spire, frequently concealed. The animal is proportionally large, slug-like, and slimy, with the power of secreting and of throwing off an adhesive mucus in copious abundance. The shell is partially covered by the mouth, the side lobes of which are well developed. The head disk is obscurely four-sided, without eyes or distinct tentacles.
THE GAPING BULLA.Of the six species of this genus which are found in the British seas, the largest is P. aperta, the Gaping Bulla. It is an unpleasing, almost shapeless slug, very soft and slimy to the touch, of an opaque white hue, sometimes tinged with pale orange. Looked at from above, it appears to be composed of four portions,—the square head-disk, the body partly enclosing the shell, and the lobes or wings of the mantle turned up on each side and investing it. It is usually about an inch and a quarter in length, but individuals are found of a larger size.
The shell, on being dislodged, is transparent and colourless, but on drying loses somewhat of its clearness, and becomes of a lustrous white hue. Its surface is smooth, except for the concentric lines, which mark its progressive increase. The aperture