Fig. 63.—Silurian Sea-snails. Canada.
a, Murchisonia bicincta (Hall). b, Pleurotomaria umbilicatula (Hall). c, Murchisonia gracilis (Hall). d, Bellerophon sulcatinus (Billings).
The highest group of Mollusks, represented in the modern ocean by the Nautili and Cuttle-fishes, has a history so strange and eventful, and so different from what might have been anticipated, that it perhaps deserves a more detailed notice, more especially as Barrande has recently directed marked attention to it in his magnificent work on the Palæontology of Bohemia.
The Cuttle-fishes and Squids and their allies are, in the modern seas, a most important group ([Fig. 64]). The great numbers in which the smaller species appear on many coasts, and the immense size and formidable character of others; their singular apparatus of arms, bearing suckers, their strange forms, and the inky secretion with which they can darken the water, have at all times attracted popular attention. The great complexity of their structures, and the fact that in many points they stand quite at the head of the invertebrates of the sea, and approach most nearly to the elevation of the true fishes, have secured to them the attention of naturalists. Some of these animals have shelly internal supports, and one genus, that of the Argonauts, or Paper Nautili, has an external protective shell. Allied, though more distantly, to the Cuttle-fishes, are the true Nautili, represented in the modern sea principally by the Pearly Nautilus, though there are two other species, both of them very rare. The modern pearly nautilus ([Fig. 65]) may be regarded as a peculiar kind of cuttle-fish provided with a discoidal shell for protection, and also for floatage. The shell is divided into a number of chambers by partitions. Of these the animal inhabits the last and largest. The others are empty, and are connected with the body of the animal only by a pipe, or siphuncle, with membranous walls and filled with fluid. Thus provided, the nautilus, when in the water, has practically no weight, and can move up or down in the sea with the greatest facility, using its sucker-bearing arms and horny beak to seize and devour the animals on which it preys. The buoyancy of the shell seems exactly adapted to the weight of the animal; and this proportion is kept up by the addition of new air-chambers as the body increases in size. In the modern seas this singular little group stands entirely isolated, and its individuals are so rare that it is difficult to procure perfect specimens for collections, though its mechanical structure and advantages for the struggle for existence seem of the highest order. But in the old world of past geological time the case was altogether different.
Fig. 64.—Squid (Loligo). | Fig. 65.—Pearly Nautilus (Nautilus pompilius). a, Mantle. b, Its dorsal fold. c, Hood. o, Eye. t, Tentacles. f, Funnel. g, Air chambers. h, Siphuncle. |
The Nautiloid shell-fishes burst suddenly upon us in the beginning of the Siluro-Cambrian, or Lower Silurian, Barrande’s second fauna; and this applies to all the countries where they have been studied. In this formation alone about 450 species are known, and in the Silurian these increase to 1,200; and here the group culminates. It returns in the Devonian to about the same number with the Lower Silurian, diminishes in the Carboniferous to 350, and in the Mesozoic, where the Nautiloid forms are replaced by others of the type of the Ammonites, becomes largely reduced. In the Tertiary there are but nineteen species, and, as already stated, in the modern world three. These statements do not, however, represent the whole truth. In the Palæozoic, in addition to the genus Nautilus, we have a great number of other genera, some with perfectly straight shells, like Orthoceras ([Fig. 66]), others bent (Cyrtoceras), others differing in the style of siphuncle, or aperture, or chambers (Endoceras, Gomphoceras, Lituites, [Figs. 67 to 69]), or inflated into sac-like forms (Ascoceras). There is, besides, the family of the Goniatidæ ([Fig. 70]), with the chambers thrown into angular folds and the siphuncle at the back. Further, some of the early forms, as the Orthoceratidæ, attain to gigantic dimensions, being six feet or more in length, and nearly a foot in diameter. Thus the idea that we should naturally form from the study of the Nautilus, that it represents a type suited for much more varied and important adaptations than those that we now see, is more than realised in those Palæozoic ages when these animals seem to have been the lords of the seas.
Fig. 66.—Orthoceras. Siluro-Cambrian. The dotted line shows the position of the siphuncle. | Fig. 67.—Gomphoceras. |
Fig. 68.—Lituites. |
