also, visible pulsations in the heart; and the mouth is armed with jaws, and with a spinous tongue. Another stage is marked by the fall of the veils, and by the budding forth of the anterior tentacula, as well as of the branchiæ; and the full evolution of these organs completes the metamorphosis and entitles the animal to the privileges of maturity.
Most of the characters which distinguish this genus have been already enumerated in those of the family. The peculiarities by which it is separated from its fellow genera are chiefly the presence of an internal shell, and the position of the gill-plume. The latter organ assumes the form of complex leaflets, attached to a broad membranous footstalk, and concealed beneath the shell.
The genus is truly marine; yet M. Rang has observed specimens of A. dolabrifera inhabiting marshes in the island of Bourbon, where the water was almost fresh, and where Neritina and Melania, both essentially fresh-water genera, were its companions.
They swim freely, by means of the large fin-like expansions of the mantle, which are waved with an undulating motion. They have been seen also floating at the surface, suspended like Pond-snails from the inverted foot.
All the species, I believe, are remarkable for the power of pouring out, in copious profusion, a fluid of a brilliant purple hue, which readily diffuses itself through the surrounding water. I have already mentioned my own experience of this phe-