Lign. 148. Fossil Shells related to the Argonaut.[408]
| Fig. | 1.— | Bellerophon costatus. Mt. Limestone. Yorkshire. |
| 2.— | Bellerophon bilobatus. Sil. Syst. |
[408] Some naturalists consider the Bellerophon to be allied to the Carinaria (Heteropod).
Bellerophon.—It has been already stated, that the animals of one genus of the existing dibranchiate Cephalopoda are protected by a thin, flexible, symmetrical, keeled shell, convoluted on a vertical plane, and having but one chamber—this is the Argonaut, or Paper Nautilus, an inhabitant of the Mediterranean. This animal belongs to the Octopoda, or those which have eight arms; and in one pair of these processes the extremities expand into broad and thin membranes, by which the delicate, elastic, calcareous envelopement, or shell, is secreted. There membranes usually encompass the shell, and meet and overlap each other along its keel; and by them chiefly the shell is retained in its position. When these membranes are withdrawn, or the animal dies, the shell, having no muscular connexion with the soft parts, readily separates from the body. Hence the doubts so long entertained as to the relation between the animal of the Argonaut and its shell, but which are now set at rest; the observations on the living animal by Madame Tower, and the anatomical demonstrations by M. Sander Rang, having removed the obscurity in which the subject was formerly involved.
In the Silurian, Devonian, and Carboniferous deposits there are several species of a genus of shells, the animals of which are by some considered to have been analogous to the recent Argonaut. It is named Bellerophon. I have figured two species; one from the Mountain Limestone, [Lign. 148], fig. 1; the other from the Silurian System. There are about thirty British species, most of which are of small size; some of them are keeled, others have a slight dorsal depression, as in fig. 1. and many have the back rounded, and the sides lobed, as in [Lign. 148], fig. 2.
FOSSIL TETRABRANCHIATE CEPHALOPODA.
I am not aware of the existence of any British fossils analogous to Spirula (dibranchiate); for the minute fossil polythalamia, formerly referred to this class, are now known to have belonged to animals possessing an organization altogether different, as we have already explained (see [p. 369]). I therefore proceed to notice the fossil remains of those Cephalopoda which were furnished with an external shell having its cavity divided by cells, which are perforated by a hydraulic tube or siphuncle; and of which group the recent Nautilus is the type.