Excerpt from Triassic Fishes of Connecticut The Cambrian system, at the base Of the Paleozoic, has thus far failed to yield any indication of the presence Of backboned creatures. Neither Chordates nor Protochordates, that is, primitive forerunners of the vertebrate phylum, make their ap pearance in the geological time-scale until the Ordovician, after which they continue sparsely throughout the Silurian. From what lower group of organisms the primitive progenitors Of the vertebrate stem were descended, and during what ...
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Excerpt from Triassic Fishes of Connecticut The Cambrian system, at the base Of the Paleozoic, has thus far failed to yield any indication of the presence Of backboned creatures. Neither Chordates nor Protochordates, that is, primitive forerunners of the vertebrate phylum, make their ap pearance in the geological time-scale until the Ordovician, after which they continue sparsely throughout the Silurian. From what lower group of organisms the primitive progenitors Of the vertebrate stem were descended, and during what period the hiatus between diverse phyla was bridged over, we have no means of knowing. The absence of transitional forms, or indeed even of Protochordates, in strata anterior to the Ordovician is not a very significant fact, when it is considered that the primitive forerunners of chordate animals were probably soft bodied, and therefore incapable of preservation in the rocks. Although numerous indications of fish-like vertebrates have been obtained at different localities both in this country and abroad, as, for instance, from the Ordovician of Colorado, Montana, and Scotland, it is not until the Silurian that their remains are found satisfactory enough for discussion. The dominant forms of fish life that we are acquainted with from rocks of this age belong to the lowly group of Ostracophores wcreatures which differ from Fishes proper to such an extent that they are usually included in a separate class (agnatha). They have incompletely formed jaws, are destitute of paired fins, and are without calcified en doskeletal parts (fig. I). On the other hand, as their name implies, they are protected by a shell - like external covering, whose elaboration can be traced through a number of successive stages. Toward the close of the Devonian they become entirely extinct, without leaving descendants. About the Publisher Forgotten Books publishes hundreds of thousands of rare and classic books. Find more at ... This book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully; any imperfections that remain are intentionally left to preserve the state of such historical works.
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