OPHIDIA. 731 



convex and continuous. Hypapophyses none on the two vertebrae pre- 

 served. Zygapophyses prominent. Free diapophyses none. 



This genus is readily distinguished by the presence, first observed here, 

 of the episplien in addition to the zygosphen, and by the pecuhar form of 

 the neural spine. We have now several vertebral articulations originally 

 discovered in American vertebrata. These are the episphen, as above; the 

 hyposphen, which characterizes the Opisthocoelous Dinosauria {Sauropocla 

 Marsh), and the Diadedidce of the Permian period ; and the zygantrapophysis, 

 which is present in the Diplocaulid family of Batrachia.^ 



Helagras prisciformis Cope. 



Proceeds. Amer. Philosoph. Society, 1883, p. 545. 

 Plate XXIVg; fig. 2. 



A section of the vertebra at the middle is pentagonal, the inferior side 

 slightly convex downwards. The lateral angle is the section of the angular 

 ridge which connects the zygapophyses. The episphen has a shallow 

 rounded groove on its infero-posterior side, which is bounded by a project- 

 ing angle on each side at its middle. The episphen does not project so far 

 posteriorly as the postzygapophyses, and the degree of its prominence dif- 

 fers in different parts of the vertebral column. In one of the two vertebrae 

 in my possession its prominence is small. The tuberosity on its summit is 

 a truncate oval with the long diameter anteroposterior, and equaling two- 

 fifths the length of the arch above. It is elevated above the rest of the 

 median line, which is roof-like, with obtuse angle. The tubercular articular 

 facet is entirely below the prezygapophysial surface, but the free part of the 

 prezygapophysis extends well in front of it. It is distinguished from the 

 capitular surface by a very slight constriction. A slight ridge extends from 

 the capitular articulation to the edge of the ball of the centrum. Below 

 this, the surface is slightly concave, and the middle line is gently convex. 

 The latter terminates in an obtuse angled mark just in front of the edge of 

 the ball. This edge is also sHghtly free from the ball. The capitular costal 

 surfaces do not project inferiorly quite to the line of the inferior surface of 

 the centrum. 



iPaleontological Bulletin No. 35, Nov. 11, 1883. 



