Lamarck constitutes many genera of the shells included in the Ostrea genus of Linnæus. His Malléacées comprehend five genera, Crenatula, Perna, Malleus, Avicula, and Meleagrina, all which are allied more or less remotely to the shell before us. To that particular family which is known among collectors by the designation of Hammer Oysters, he gives the name of Malleus, in the French Marteau, both alike implying the hammer like form of the species Malleus, which Lamarck assumes as the type of this genus. But even there after all the renovation that has been attempted, the result is not satisfactory, because this figure is by no means constant, even in the few species included by its author in that genus; it contains but six species, and these are entirely at variance with each other. Thus for example, in Malleus Vulgaris, the common Hammer Shell, we have a species with three lobes, a lateral one of considerable size being advanced on each side the beaks: and another shell of the same species with only short lateral lobes instead of large ones. Admitting the hammer form to be still preserved in these, in the next species, Malleus Normalis, instead of two lobes, the hammer head, if it may be so expressed, has but a single lobe: in Malleus Anatinus there is only one lobe, and that very small; and in Malleus Vulsellatus, although characterised as “_lobo oblique porrecto_,” the appearance of the shell implies rather the total absence of any lobe, for the lobe, if so it may be termed, is so indefinite, that it cannot be referred without violence to the genus Marteau, while we consider its hammer like form as a leading character of the genus. With exception to this inconstant character which may be qualified with the expression “deformed and generally hammer shaped,” we have no objection to the Malleus genus, because the byssus of the animal by means of which it can affix itself to other bodies, and the peculiar sinus or sulcation of the hinge through which the byssus passes from the animal to those extraneous bodies, are sufficient to remove it from the Ostrea genus, in which case if we still adhere to the Linnæan method we can place it only among his Mytili or Pinnæ, and it has certainly less affinity with either of those than with Ostrea. Perhaps the name of Perna under which this shell has been mentioned a few years ago might have been as well preserved, but that name Lamarck assigns to an extensive genus of which Ostrea Isognomum is the type, and it is therefore better to retain the name Malleus than to alter it to another which could not fail at this time to create confusion. The same consequence would as unquestionably result were we to sub-divide the Malléacées into different genera according to the configuration of the shell or number of its lateral lobes.