In our last number we defined what appear to be the leading forms among the _Strombii_, from such characters as are exhibited by their shells. This mode of classification, we admit, is always attended with hazard; and should not be ventured upon without due precaution. To attempt a _natural_ arrangement of the higher groups, on such considerations alone, would be manifestly absurd. On the other hand, we think both principles may be carried too far: and that a system founded exclusively upon the animal would, in our present state of knowledge, be little less artificial than another taken merely from the shell. We see, on every side, throughout Nature, the most beautiful and perfect adaptation of structure to use. The typical _Mollusca_, whether among the _Acephala_ or _Gasteropoda_, appear to be testaceous; and to exhibit a corresponding perfection in the structure of their coverings: one form passes into another by as gradual modifications of the shell, as could possibly be looked for in the nature of the animal. We have attempted to exemplify this among the _Unionidæ_ and the _Olivæ_. We shall now examine the _Strombii_ with reference to the same object. "_Les Aileés_," observes Lamarck, "_constituent une famille très-naturelle, qui avoisine celle des Canaliferès par ses rapports, mais qui en est éminemment distincte_." This able classifier (whose perception of affinities is truly admirable), then proceeds to divide these shells into three groups, founded upon such excellent distinctions, that we need not here dilate on their importance. Two of these genera are characterized by a deep and well defined sinus or notch, towards the base of the outer lip; quite distinct from the basal canal, and intended either for the passage of the animal's proboscis, or its respiratory siphon. In _Strombus_ the outer lip is dilated, but entire; in _Pteroceras_, it is equally enlarged, but divided into long processes. Such are the typical distinctions, and their immediate union appears effected by the _Strombus laciniatus_ of Chemnitz. The third genus of Lamarck's is _Rostellaria_, in which he places all those species wherein the sinus above mentioned is united to the basal canal. Now if the _Strombus pespelecani_ of authors had this canal more closed, and the sinus distinct, it would be a _Pteroceras_. Da Costa long ago placed this shell as the type of a genus, and the observations of Mr. Dillwyn, in his short but highly valuable paper on fossil shells (Phil. Trans. for 1823, p. 1, p. 393), have shewed the importance which we should attach to its structure: it is, in fact, a _Pteroceras_, but with the above mentioned sinus removed to the base of the digitated lip, and confounded, as it were, with the basal canal. Excepting to a _Pteroceras_, it cannot be compared with any known shell, unless it be to the _Strombus fissus_ of Linn. The hiatus between them certainly appears great, yet the affinity is not