Page:Popular Science Monthly Volume 21.djvu/782

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ress toward that position. The history of their past begins with the recognition of the pearly nautilus (Fig. 3) as a being which, as a four-gilled cuttle-fish possessing an external many-chambered shell, stands alone in the world of life. It is the tribes of two-gilled cuttle-fishesPSM V21 D782 Spirula.jpgFig. 6.—Spirula. which people our ocean to-day, and which exhibit ail the gradations of form and size, from the minute Spirula (Fig. 6) to the great Architeuthis of the American coasts. The history of the cuttle-fishes in time begins in the far-back epoch represented by the Lower Silurian rocks of the geologist. There are entombed the first fossil cuttle-fishes, represented by their chambered shells. The genus Orthoceras, represented by shells of straight form, is thus among the oldest members of the cuttle-fish race. The Nautilus genus itself begins in the Upper Silurian rocks; we may trace the well-known shells upward to the Carboniferous strata, where they are best developed; and we follow the genus onward in time, as it decreases in numbers, until we arrive at the existing order of things, in which the solitary nautilus remains, as we have seen, to represent in itself the fullness of cephalopod life in the oceans of the past. The older or Palæozoic rocks reveal a literal wealth of these chambered shells, and therefore of the existence of the four-gilled cuttle-fishes as the founders of the race. When we ascend to the Mesozoic rocks (ranging from the Trias to the Chalk), we meet with new types of the chambered shells well-nigh unknown in the Palæozoic period. In the Mesozoic rocks appears the fullness of Ammonite life. Here we find shells named after the horns of the Egyptian god, Jupiter Ammon; these, instead of being tolerably plain, like the Nautilidœ, exhibit beautifully sculptured outlines, and folded septa, or partitions, between the chambers of the shell. The shells allied to Nautilus and occurring in the Palæozoic formations differ from Nautilus chiefly in their varying degrees of curvature or straightness. Lituites is a curved form allied to Nautilus / while Orthoceras and Gomphoceras are groups representing the straightened forms. But in the Silurian period more complex forms appear, with elaborate and folded septa. These are the early Ammonites, such as Goniatites and Bactrites. In the Secondary rocks we find the still more complex true Ammonites themselves. Here the lobes and saddles of the shells, as the edges of the septa are named, are of the most elaborate patterns, while the shapes of shell are of the most varied character (Baculites, Turrilites, Ammonites, etc., Fig. 5).

There is thus an advance and progression exhibited in the development of the four-gilled races which accords perfectly with the theory of evolution and descent. The seas of the Trias, Oolite, and Chalk periods must have literally swarmed with these striking forms of cephalopod life; but as the close of the Chalk period dawned, and as