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. 



Fig. 25. — Ophiacodon mirus Marsh. Seventh to twentieth vertebrae from the side, X yi- 



The centra of the cervical vertebrae 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 

 vertebra; 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 vertebras are 

 of nearly imiform height throughout the presacral series, ascending above the 

 centra about 2.5 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 outline. Back of the eleventh or 

 twelfth vertebra the spines decrease in thickness and increase in width, becoming 

 quite thin and flat in the posterior region, as shown by the outline figures of the 

 extremities. Very remarkably the fourth and the ninth spines, especially the fourth, 



