REPTILIA: VARANOSAURUS 93 



block of the matrix as collected had been broken off and lost, some 

 six or seven in all; four of these have been replaced in the skeleton 

 by corresponding vertebrae of another skeleton, but the figures 

 given are those of the single specimen. This series is composed 

 of twenty-seven presacral, two sacral, and forty-seven caudal 

 vertebrae, to which may be added a few, perhaps half a dozen, 

 minute terminal ones that were lost. Of the atlas, only the odon- 

 toid could be recovered from the matrix; the arches were so im- 

 pressed upon a fragment of the occipital region of the skull that 

 they could not be separated. The figure in the plate, together 

 with others of a closely allied form from the Craddock bone-bed 

 (Plate VII), will show the shape of this bone sufficiently well I 

 hope. Its posterior surface is deeply concave, in apposition with 

 the centrum of the axis; the perforation continues through the 

 bone with a small aperture in front, which coincided with a small 

 pit in the end of the occipital condyle. The upper surface of the 

 odontoid is slightly concave for the floor of the neural canal. The 

 bone in the middle reaches the ventral side, separating the atlantal 

 and axial intercentra. The surface for the atlantal intercentrum 

 is a little larger than that for the axial. On either side in front 

 there is a somewhat oblique surface for the articulation of the atlan- 

 tal arch. In the plate I give the outlines of the basioccipital and 

 atlantal intercentrum found attached in another skeleton of the same 

 species. The atlantal intercentrum, it is seen, is relatively small, 

 not much larger than the axial, with only small attachment for the 

 arch, which must have rested for the most part, if not entirely, 

 upon the odontoid or pleurocentra, as in Poecilospondylus Case. 

 It is very evident that we have to do here, as in Dimetrodon, with 

 a primitive condition of the atlas, a condition in which it differs 

 very slightly from the ordinary vertebra, one in which the atlantal 

 arch is supported almost wholly by the pleurocentra, articulating 

 not only by the usual zygopophysial way with the axis, but appar- 

 ently with the exoccipitals as well, as shown in Dimetrodon, by an 

 articular surface on either side of the foramen magnum. These 

 may be for an unrecognized proatlas. Several very perfect atlantal 

 arches are preserved among the Craddock bone-bed material, 

 which will be figured and discussed elsewhere. 



