. Elements of geology : a text-book for colleges and for the general reader. Geology. 300 PALAEOZOIC SYSTEM OF ROCKS. est to the geologist because it marks the outline of the earliest Silurian sea, and contains the remains of the earlest Silurian fauna. Indeed, we may say it contains the remains of the earliest known fauna. It is true, the lowest Ehizopods probably existed in Archaean times, but these can not be said to constitute a fauna. With the very commence- ment of Silurian times, however, we find at once a considerable variety of animal forms. What, then, was the character of this earli
. Elements of geology : a text-book for colleges and for the general reader. Geology. 300 PALAEOZOIC SYSTEM OF ROCKS. est to the geologist because it marks the outline of the earliest Silurian sea, and contains the remains of the earlest Silurian fauna. Indeed, we may say it contains the remains of the earliest known fauna. It is true, the lowest Ehizopods probably existed in Archaean times, but these can not be said to constitute a fauna. With the very commence- ment of Silurian times, however, we find at once a considerable variety of animal forms. What, then, was the character of this earliest fauna and flora ? If we could have walked along that beach when it was washed by pri- mordial seas, what would we have found cast ashore ? We would have found the representatives of all the great types of animals except the vertebrata. The Protozoa were then represented by sponges and Ehizopods; the Eadiates by Hydrozoa (graptolites) (Fig. 276) and Cys- tidean Crinoids; the mollusks by Brachiopods, Lamellibranchs, Gas- teropods (Pleurotomaria), Pteropods (Figs. 272-279), and even Cepha- lopods (orthoceras) ; and the Articulates by Crustaceans (trilobites, etc.) (Figs. 269-271) and Worms (Fig. 281). Plants are represented by Fucoids. These widely-distinct classes are already clearly differentiated and somewhat highly organized. Nor is the fauna a meager one in number of species. In the United States and Canada alone about 400 species are already known in the primordial, of which nearly 100 are trilobites; and in the lowest zone of the primordial, viz., Olenellus beds, there are 134 species, of which 55 are trilobites (Walcott). About. Fig. 280. Fig. 283. Fig. 286. Fig. 288. Figs. 280-288.—Foreign Primordial Fossils: 280. Oldliamia antiqua, probably a plant. 281. Arenicolites didymns, worm-tubes. 282. Lingulella ferruginea. 283. Theca Davidii. 284. Modiolopsis solvensis. 2^5. Ortliis Hicksii. 286. Obolella sagittalis. 287. Hymenocaris vermi- cauda. 288. Olenus macrurus. a d
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectgeology, bookyear1892
