The conformation of this shell is very striking, and yet we perceive that its essential characteristics are less definitive than could be wished; there are several approximations in the general figure to be found among shells which nevertheless possess characters generically distinct. For many years this shell was known in this country under the name of “_Margaritifera maculata_,” and the trivial English appellation of the “_Spotted Hound’s Tongue_:” it appeared under those names in the Conchological Museum of M. de Calonne, while it remained in England, and in the catalogue of that museum, which is still extant, it will be found under those names. The epithet of Hound’s Tongue is not inaptly applied to this shell, in allusion to the elongated form. The term Margaritifera does not refer to the form, but to the pearly gloss that appears upon the surface of the dark blue space lying within the shell, immediately below the hinge, and extending from thence about one fourth part of its whole length. This is the region in which the animal is attached by its ligament to the valves of the shell; besides which, a gloss of pearly hue is observed to pervade the whole of the inner surface, only that it is most conspicuous in the darker disk of the shell. As a secondary character this pearliness is very remarkable in the shell before us, at the same time that as a generical denomination the term Margaritifera assigned to it from this circumstance alone is liable to objection; because, the same pearliness prevails in many shells which have no relation whatever with the present, either in the form or structure of the hinge, and it is to these we must resort for its true essential character.