a oie 
‘that vertebra. The lateral margin below the 
axis is deeply concave, the surface) slopes in 
GILMORE: OSTEOLOGY OF BAPTANODON (MARSH) 103 
neural canal and the surfaces for the attachment of the pedicles of the neural arch, 
the two surfaces being separated by narrow antero-posterior ridges. 
The lateral surface of the combined centra (see fig. 10) is somewhat concave 
antero-posteriorly. Superiorly the neurapophysial surface extends outward and 
-coalesces with the heavy articular diapophysis. There is no indication of a dia- 
pophysial surface on the axis such as is seen in the axis of B. marshi (fig. 26), or the 
axis of I. longifrons, Owen. Just below the diapophysis and separated from it by a 
non-articular tract is an antero-posterior ridge at either end of which arise articular 
prominences which represent the parapophysis 
of the atlas and axis respectively. The parapo- 
physis of the atlas (p.) being the larger of the 
two. The round tubercle like parapophysis (p.’) 
of the axis is placed well posterior on the side of 
parapophysis is concave antero-posteriorly even 
down to the furthermost extent of the inferior 
subtriangular part of the centra. I have ex- 
amined the coalesced centra pertaining to six 
different Ichthyosaurian skeletons from the Ju- 
rassic of North America and have yet to find a 
trace of the suture uniting the atlas and axis. 
Posteriorly (see fig. 11) the centrum of the 
gradually from the outer margin for half of the 
radius, thence more sharply to the center. In- 
feriorly the lower surface is produced into a sub- Fic. 11. Posterior view of the atlas and 
triangular face which looks downward and back- axis of Baptanodon discus (No. 878). One half 
ward, at a somewhat sharper angle (see fig. 10 Se ame une Spit cde ee 
neural arch of the atlas; p., parapophysis of 
than the apophysis of the atlas. This would be atias; s., spine of the axis; zyg., posterior 
the articulation for the third intercentrum if ygapophysis of the axis; z.’, apophysis for 
that bone still exists as a separate element in pia ee can a 
this form. The axis of B. marshi, shows no such face, though the apophysis of the 
atlas appears to still be present. The axes of both No. 603 and 919 appear to be 
identical with that of No. 878. In these forms it would seem that at least two 
intercentra exist as separate elements while the one between the atlas and axis has 
become completely anchylosed to the centra. I haye examined the atlas and axis 
of B. marshi (see fig. 26) and find here a still greater reduction in the number of 
