16 OSTEOLOGY OF PTERANODON. 



" The centrum is elongated. The ball is much broader than high, and strongly con- 

 vex in both directions ; its upper border is convex, but the inferior margin is emargi- 

 nate on each side. . . . there is a stout process, jutting downwards and backwards 

 from the side of the centrum on each side, having on the posterior surface a concave 

 articular facet, oval in shape and touching or slightly separated from the articular sur- 

 face of the ball. The facet looks downwards, backwards, and outwards. . . . The 

 corresponding articular facets on the anterior end of the centrum are somewhat smaller, 

 are convex and distinctly separated from the concavity of the centrum. The artic- 

 ulation of the centra with each other thus depends upon three distinct, or nearly 

 distinct surfaces, the lateral inferior ones convex on the cup end, concave on the 

 ball end. Such a mode of articulation would seem to limit the motion to one in 

 a vertical, antero-posterior plane, while greatly strengthening the joints. I know of no 

 similar arrangement in any other vertebrate animal, and will, for convenience, call the 

 articulations exapophyses. The anterior zygapophyses project distinctly beyond the 

 plane of the cup and are widely separated from each other. From the tip of the proc- 

 esses a ridge runs downward and inward to the outer part of the pre-exapophyses. 

 The post-zygapophyses are concave and oblique. Above them there is a stout met- 

 apophysis. 1 The spine is elongate and thin, and apparently only a ridge." 



The elongated cervicals all possess distinct tubercular hypapophyses, which project 

 from the anterior ends of the centra beneath the oval articular cups. The vertebrae are 

 lightened by pneumatic canals, which enter the centra midway of the sides, and 

 also by paired canals that under favorable conditions of preservation may be seen at 

 both ends a little above the level of the neural canal. The first seven cervicals are 

 without ribs. 



The eighth and ninth vertebrae resemble each other closely, but differ greatly from 

 those preceding them. Their form is well preserved in Pteranodon sp., No. 2692 (Plate VI, 

 figures 8 and 9, figure 10 portraying diagrammatically the anterior view of the eighth 

 cervical). The centra are short, their length and breadth being nearly equal. The artic- 

 ular facets of the prezygapophyses and preexapophyses remain much the same as in the 

 elongated cervicals, but their basal portions are produced laterally to form transverse 

 and capitular processes bearing ribs. The ribs of the eighth cervical somewhat resemble 

 the hatchet ribs characteristic of these vertebrae in certain other groups of Reptilia, but 

 they are little produced anteriorly and posteriorly. They are firmly coossified with the 

 supporting processes. The right rib of the ninth cervical is of the usual double-headed 

 form (Plate VI, figure 16). Its diameter is comparatively small and its length is only 

 about one-half that of the first dorsal ribs. Evidently it did not reach and connect with the 

 sternum. A pair of short double-headed ribs found with P. ingens, No. 1175, the right 

 member of which is shown in Plate VI, figure 15, probably belonged to the missing 

 ninth cervical. From the inferior margin of the articular cup of the eighth cervical pro- 

 jects a small hypapophysis. It is slightly bifid in two examples in the collection ; in 

 others, both large and small, the hypapophysis is simple, but because of the incomplete- 

 ness of the types, it is impossible to make use of these differences as specific charac- 



1 In this instance, Professor Williston evidently uses the term metapophysis in a general sense, 

 just as he does in his paper on the Osteology of Nyctosaurus. Another authority informs us that 

 we must apply this term only to the "apophysis developed on the prezygapophysis or anterior 

 articulating process of a vertebra." 



