. Carnegie Institution of Washington publication. PERMO-CARBONIFEROUS VERTEBRATES FROM NEW MEXICO. 45 poliosaurids. The spine is thickened behind, thin in front, projecting much an- teriorly, and is helmet-shaped in outline. The atlantal zygapophyses in front are relatively small. The centrum differs very little from that immediately following. Postaxial vertebrce (figs. 24-26): The centra of the first four vertebrae back of the atlas are especially characterized by the obliquity of their articular faces; this obliquity decreases on the fifth, and the centra of the following vertebrae all have
. Carnegie Institution of Washington publication. PERMO-CARBONIFEROUS VERTEBRATES FROM NEW MEXICO. 45 poliosaurids. The spine is thickened behind, thin in front, projecting much an- teriorly, and is helmet-shaped in outline. The atlantal zygapophyses in front are relatively small. The centrum differs very little from that immediately following. Postaxial vertebrce (figs. 24-26): The centra of the first four vertebrae back of the atlas are especially characterized by the obliquity of their articular faces; this obliquity decreases on the fifth, and the centra of the following vertebrae all have their terminal articular faces nearly at right angles to their long diameters. On the first four postaxial centra the cartilaginous surface of the anterior rims is carried far back. The capitula of the ribs articulated in part on this surface, but its great extent is doubtless due to the increased mobility of the vertebrae in this region. For this reason, as also because of the position of the scapulae in the specimen, the first six, if not the first seven, vertebrae may be considered as true cervicals. O O. 11 1, ' 1' Fig. 25.—Ophiaiodon iiiinis Marsh. Seventh to twentieth vertebne from the side, X K- The centra of the cervical vertebra are a trifle longer than those of the dorsal region, which are all nearly uniform in length. The under side is gently concave in outline. The anterior vertebras have a sharp, thin keel below; in the posterior vertebras the keel is thicker, with angular or parallel sides. The articular faces of the zygapophyses are directed at a considerable angle inward and outward. The zygapophyses are strongest in the cervical region. The spines of the vertebra; are of nearly uniform height throughout the presacral series, ascending above the centra about times their vertical diameter, or a little more. Anteriorly, as far as the tenth or eleventh vertebra, they are stout, with a truncate, cartilaginous surface at the extremity, of an oval or ovate
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