Lastly, the Lower Silurian Rocks have yielded a vast number of chambered shells, referable to animals which belong to the same great division as the Cuttle-fishes (the Cephalopoda), and of which the Pearly Nautilus is the only living representative at the present day. In this group of Cephalopods the animal
Fig. 53.—Different views of Bellerophon Argo, Trenton Limestone, Canada. (After Billings.) possesses a well-developed external shell, which is divided into chambers by shelly partitions ("septa"). The animal lives in the last-formed and largest chamber of the shell, to which it is organically connected by muscular attachments. The head is furnished with long muscular processes or "arms," and can be
Fig. 54.—Different views of Maclurea crenulata, Quebec Group, Newfoundland. (After Billings.) protruded from the mouth of the shell at will, or again withdrawn within it. We learn, also, from the Pearly Nautilus, that these animals must have possessed two pairs of breathing organs or "gills;" hence all these forms are grouped together under the name of the "Tetrabranchiate" Cephalopods (Gr. tetra, four; bragchia, gill). On the other hand, the ordinary Cuttle-fishes and Calamaries either possess an internal skeleton, or if they have an external shell, it is not chambered; their "arms" are furnished with powerful organs of adhesion in the form of suckers; and they possess only a single pair of gills. For this last reason they are termed the "Dibranchiate" Cephalopods (Gr. dis, twice; bragchia, gill). No trace of the true Cuttle-fishes has yet been found in Lower Silurian deposits; but the Tetrabranchiate group is represented by a great number of forms, sometimes of great size. The principal Lower Silurian genus is the well-known and widely-distributed Orthoceras (fig. 55). The shell in this genus agrees with that of the existing Pearly Nautilus, in consisting of numerous chambers separated by shelly partitions (or septa), the latter being perforated by a tube which runs the whole length of the shell after the last chamber, and is known as the "siphuncle" (fig. 56, s). The last chamber formed is the largest, and in it the animal lives. The chambers behind this are apparently filled with some gas secreted by the animal itself; and these are supposed to act as a kind of float, enabling the creature to move with ease under the weight of its shell. The various air-chambers, though the siphuncle passes through them, have no direct connection with one another; and it is believed that the animal has the power of slightly altering its specific gravity, and thus of rising or sinking in the water by driving additional fluid into the siphuncle or partially emptying it. The Orthoceras further agrees with the Pearly Nautilus in the fact that the partitions or septa separating the different air-chambers are
Fig. 55.—Fragment of Orthoceras crebriseptum, Cincinnati Group, North America, of the natural size. The lower figure section showing the air-chambers, and the form and position of the siphuncle. (After Billings.)
Fig. 56.—[14] Restoration of Orthoceras, the shell being supposed to be divided vertically, and only its upper part being shown. a, Arms; f, Muscular tube ("funnel") by which water is expelled from the mantle-chamber; c, Air-chambers; s, Siphuncle. simple and smooth, concave in front and convex behind, and devoid of the elaborate lobation which they exhibit in the Ammonites; whilst the siphuncle pierces the septa either in the centre or near it. In the Nautilus, however, the shell is coiled into a flat spiral; whereas in Orthoceras the shell is a straight, longer or shorter cone, tapering behind, and gradually expanding towards its mouth in front. The chief objections to the belief that the animal of the Orthoceras was essentially like that of the Pearly Nautilus are—the comparatively small size of the body-chamber, the often contracted aperture of the mouth, and the enormous size of some specimens of the shell. Thus, some Orthocerata have been discovered measuring ten or twelve feet in length, with a diameter of a foot at the larger extremity. These colossal dimensions certainly make it difficult to imagine that the comparatively small body-chamber could have held an animal large enough to move a load so ponderous as its own shell. To some, this difficulty has appeared so great that they prefer to believe that the Orthoceras did not live in its shell at all, but that its shell was an internal skeleton similar to what we shall find to exist in many of the true Cuttle-fishes. There is something to be said in favour of this view, but it would compel us to believe in the existence in Lower Silurian times of Cuttle-fishes fully equal in size to the giant "Kraken" of fable. It need only be added in this connection that the Lower Silurian rocks have yielded the remains of many other Tetrabranchiate Cephalopods besides Orthoceras. Some of these belong to Cyrtoceras, which only differs from Orthoceras in the bow-shaped form of the shell; others belong to Phragmoceras, Lituites, &c.; and, lastly; we have true Nautili, with their spiral shells, closely resembling the existing Pearly Nautilus.
[Footnote 14: This illustration is taken from a rough sketch made by the author many years ago, but he is unable to say from what original source it was copied.]