dimensions. It is much shorter in proportion than the common species, seldom exceeding an inch and a half or two inches in length. It is, however, very stout in proportion. A very fine specimen now before me exhibits beautifully the single accessory valve, with its curious markings, as well as the interior hinge-projections, and the bold tooth-like projections of the shell, which are so close and so deeply waved towards the base of the shell, that they give peculiar richness of effect.
At Plate I., fig. 6, is represented a specimen of the Little Piddock, imbedded in the red chalk of Brighton.
The White Piddock has also a single accessory valve, which is broader and not so pointed as that of the preceding species. It is another of the small Piddocks, being about the same size as the Little Piddock, and is even more fragile than the generality of its fragile kin. All these shells require the most careful handling, for not only are the shells themselves delicate and brittle, but the accessory valves are so thin, so easily broken, and so slightly attached, that a rude grasp is sure to crush them, or at all events to break them away from their attachments: This species is comparatively active, readily taking alarm, and instantly retreating into the depths of its tunnel.
There is a genus of shells which are closely allied to the Piddocks, and which, in the opinion of several conchologists, really ought not to have been separated from them. This genus is called Pholaididea, and a figure of the shell and animal may be seen at Plate I. fig. 10.
The reader will at once note the remarkable cup-like appendage at the feet of the siphons, which affords the chief reason why the creature has been placed in a separate genus. The cup does not seem to discharge any particular office; but Messrs. Forbes and Hanley think that it is the germ of a supplementary tube, which, if elongated, would cover and protect the
