The definition of Malleus in the _Règne Animal_ of Cuvier appears to intimate the same objection; it does not consider the hammer like form of the shell as any criterion, it is only stated that the Marteaux are inequivalve and irregular, that they have a simple hollow for the ligament as in the oysters, but that they are distinguished by a slope at the side of the ligament for the passage of the byssus. It is assuredly true that the presence of a byssus in this tribe of shells displaces them from any immediate analogy with the Ostrea, where as Cuvier remarks “Linnæus left them.” But, if however, we attentively examine the hinge of the common oyster, the two valves, and the oyster as it lies within the valves, we shall perceive with this exception a pretty near approximation. The great objection is, that the animal of the tribe of shells now before us protrudes a byssus from its body through a lateral opening on one side or slope of the ligament of the hinge; if we closely inspect the valves of the oyster, we also find a slight depression or hollow upon each side of the cartilage of the hinge; these are small, and usually somewhat lamellar. The oyster, moreover, as it lies in the shell, seems capable of expanding or spreading that part of the body which lies under the hinge laterally upon and into these depressions, a circumstance very easily observed in the half famished oyster, because these lateral expansions of the animal are then more visibly elongated along the passage of these lateral grooves of the hinge, and give the pointed end of the animal a somewhat cornuted appearance. Under the same circumstance these processes adhere as they lie in the hollow of these grooves, and thus suggests the idea of the animal having exerted itself by such extension to obtain refreshment through these lateral hollows. Those hollows are also so far pervious as to admit the ingress of moisture while the shells are closed, in the same manner as it is possible the Malleus genus may receive moisture under the same circumstance through the sinus, whence the byssus is protruded. These peculiarities considered, may perhaps afford some further justification of Linnæus in placing the hammer shells with the Ostreæ. It has been indeed advanced that Linnæus was not aware of these hammer shells being furnished with a byssus, or that he would have referred them to the Mytili, but this observation cannot be correct, because in the figure given of these shells by Seba, to which Linnæus refers, the byssus, which is very conspicuous, is represented pendent or hanging to a considerable length out of the shell.