File:Image from page 41 of "Water reptiles of the past and present" (1914) (14772637352).jpg

Identifier: waterreptilesofp1914will Title: Water reptiles of the past and present Year: 1914 (1910s) Authors: Williston, Samuel Wendell, 1851-1918 Subjects: Aquatic reptiles Publisher: Chicago, Ill., The University of Chicago Press Contributing Library: Boston Public Library Digitizing Sponsor: Boston Public Library

View Book Page: Book Viewer About This Book: Catalog Entry View All Images: All Images From Book

Click here to view book online to see this illustration in context in a browseable online version of this book.

Text Appearing Before Image: gments, or vertebrae, ossify about this rod in allreptiles, forming bony rings, perforate at first in the middle forthe more or less constricted notochord. This stage was thepermanent condition in all the earliest reptiles and in some laterones. Such animals are said to have notochordal vertebrae, the 3° WATER REPTILES OF THE PAST AND PRESENT notochord more or less continuous, like a string of beads, the beadsrepresenting the enlargements between the contiguous vertebrae.In many early amphibians, and probably in all the earliest ones,as well as in the fishes from which they were derived, the vertebrais more complicated in that it is composed of at least three pairsof separate bones, two of which united with each other, the thirdfinally disappearing in modern animals, or at the most representedby a mere vestige called the intercentrum. The dorsal pair ofthese bones, called the neurocentra, forms the arch of the vertebra.The ventral posterior pair, called the pleurocentra, increases in

Text Appearing After Image: Fig. 13.—Notochordal cervical vertebrae, with intercentra, of Ophiacodon, aprimitive theromorph reptile from the Permocarboniferous of New Mexico: pa, pro-atlas; an, arch of atlas; 0, odontoid; ax, axis. size and unites to form the centrum or body of the vertebra; whilethe ventral anterior pair, early united with each other, is called thehypocentrum or intercentrum, persistent in all early reptiles asa vestige between the centra on the ventral side. This dividedcondition of the vertebra is persistent in the first vertebra, theatlas of all higher animals, in which the so-called body is thehypocentrum or intercentrum, the arch is the neurocentrum,while the pleurocentra have fused more or less with the anteriorpart of the next vertebra, the axis, to form the so-called odontoid.That this is the real explanation of the structure of the atlas THE SKELETON OF REPTILES 31 is proved by the various stages of its evolution in the reptiles,from the earliest (Fig. 15) in which it scarcely differ

Note About Images