32 BULLETIN" 110, UNITED STATES NATIONAL MUSEUM. 



end of the pedicle, the greater expansion of this end antero-posteriorly, and especi- 

 ally in the anterior position of the posterior zygapophyses. As in the latter genus 

 the vertical concave surface of the pedicle contributes to the formation of the cup 

 for the condyle of the skull. Above the antero-posterior constriction the neura- 

 pophysis widens into a thin mesially directed plate, the superior surface of which 

 looks more upward than outward. The anterior portion is directed inward and in 

 conjunction with the process of the opposite side forms the covering for tlio neural 

 arch through which the spinal cord passes from the foramen magnum. The pos- 

 terior e.xtension is largely missing in this specimen, but doubtless it will be found to 

 extend backward as a long tapering process as in Ceratosintrus (fig. 4, pi. 19). 



The posterior zygapophysis on account of its anterior position is not visible 

 from a lateral view as it is in Ccraiosavrus. This difTerence in ]iosition may be clearly 

 seen bj' comparing figure 17 and figure 4, plate 19. 



The odontoid is free in the present individual, though it becomes fused later in 

 life, as indicated by the rugose nature of the posterior articular surface, and as 

 shown in specimen No. 8367, U..S.X.j\I. (fig. 18, o), which has the odontoid iirmly 

 fused with the axis. It is wider than long (.54 mm. wide and 32 mm. long) ; the 

 upjier surface slightly concave from side to side ; viewed from the front the anterior 

 protuberance that rests in the superior concave surface of the intercentrum is reni- 

 form in outlme. 



On the median anterior end there is a sulicircular deju-ession whose surfaces 

 conform closely with the general concave cup of the other articidated elements of 

 the atlas. Its posterior ventral border rests ujmn the su]ierior surface of the axis 

 hypocentrum. 



Axis (ax.). — The axis, although having suffered slight distortion, is neverthe- 

 less most perfectly preserved. The centrum is planoconcave, with rounded ventral 

 surface without keel (fig. 17, C). In transverse diameter the ends are exjanded, 

 the median lateral sides much pinched in, with a small elongated opening tcnvard 

 the anterior end. The neural arch is comparatively low and heavy, being sur- 

 mounted by a long, strongly developed median crest that rises rapidly toward the 

 back. The posterior portion flares into a transversely exj^anded, overhangmg 

 spine. There are deep notches between the sides of this spine and the posteriorly 

 projecting zygajiophyses, whicli distinguishes at once the axis of Antrodemus from 

 that of Ceratosavrus with its little difTerentiated posterior zygapophyses. The pos- 

 terior zygapophyses almost entirely overhang the end of the centrum. Inferiorly 

 they present very large subcircular articular faces that look almost directly down- 

 ward. Between them a deep cavity extends forward into the arch above the neural 

 canal. Obtuse roughened protuberances extend backward and upward from the 

 top of each zygapophysial process slightly overhanging the underlying articular 

 surfaces. 



The anterior zygapophyses are rather weakly developed, consisting of rather 

 thin pointed projections that extend forward and outward from the neural arch, 

 considerably below the level of the anterior part of the metlian crest. They over- 

 hang slightly the anterior end of the centrum. Tlie anterior end of the median 

 crest is notched beneath and ends forward as a hea^'y obtuse projection, that over- 



