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Page:Popular Science Monthly Volume 72.djvu/570

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POPULAR SCIENCE MONTHLY

here in older rocks than in Europe. Among the many forms known may be mentioned some with a peculiar type of vertebra in several allied genera found in Ireland, western Europe and North America; some with, fish-like scales over the whole body found in Ireland and Ohio; the presence of the snake-like forms in Ireland, England, western Europe and Ohio and the presence on the continents of Europe and North America of the Branchiosauria already referred to. All of the types of amphibians possess the peculiar ventral armature which was arranged in a chevron pattern over the belly, breast, throat, and even extended, in some cases, out on the limbs.

The earliest quadrupeds, as we know them, form highly specialized groups of organisms which had become differentiated into five distinct lines by the close of the Carboniferous. We must await the progress of discovery to unfold for us the nature of the most primitive quadrupeds from which the earliest known forms have sprung.