Zoological Illustrations Series II/Plate 39



We first recorded this singular-shaped Strombus from a specimen sent from the Coasts of Peru. Others, from the opposite side of the American Isthmus, collected near Panama, have more recently come into the possession of our friend, the Rev. J. Bulwer, F.L.S. Its general form is not unlike S. gallus, but the spire is unusually depressed, and the aperture deeply striated: the upper surface is marked by parallel grooves, and one of the nodules is particularly large.

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 interrupted by any thing known: this latter shell resembles the true Rostellariæ, but with the digitated processes of Pteroceras reduced to teeth; which teeth, in R. curvirostris, become confined to the basal part of the unexpanded lip. We are here met by certain fossil shells, preserving the form of Rostellaria, but with the outer lip entire, and sometimes considerably expanded: in these the spiral canal sometimes extends to the summit of the shell: finally the distinct sinus again appears in such species as S. cancellatus, canalus, &c., all which have been classed by conchologists with Strombus. This series is sufficient to shew the tendency of a circular disposition of relations, but very many of the intermediate forms are wanting, nor is it at all clear in what way the immediate passage is effected between Hippochrenes and the genuine Strombii.