have surrounded the aperture, and meet each other on the outer lip. Here the meeting edges unite by mutual adhesion, and seem to grow together, yet the suture is always distinctly visible, both by a slight depression, and by a pale line which assumes a zigzag form, owing to the terminations of the body striæ fitting into the interspaces of the opposite ones.
What is curious in the case is the instinct which makes the Adamsia select a shell as its constant support, and the association with it of a Hermit Crab as the co-tenant of the same shell. This association is I think constant; for though the dredge does occasionally bring up shells invested by the Adamsia, which are empty, yet I incline to believe that these shells have been recently vacated by the tenant Crabs, and not that they have never been so occupied at all.
That the above is the correct explanation is evident from specimens in various stages of development. There is in my possession, while writing this note, an Adamsia, adhering to a Whelk, of which the lateral lobes, though projected around the edges of the mouth of the shell, have not yet met each other on the outer lip, but are separated by a space of a quarter of an inch. And my friend Mr. Thompson, whose opportunities for studying the marine animals of Dorsetshire, have been most zealously improved, has just showed me a very young specimen, not larger than a silver three-pence, in which the side lobes were not in the least developed. This specimen had selected a land shell as its support, a not quite adult Garden Snail (Helix aspersa), within which a Pagurus Prideauxii had
