48 



MARINE REPTILES OF THE OXFORD CLAY. 



bone (w.b. 1), the anterior face of which is concave and forms the lower part of the 

 atlantal cup ; its upper ends unite in straight sutures with the lower ends of the 

 bases of the atlantal neural arch (at.a.), which unite with the centrum of the atlas in a 

 large nearly triangular facet, which looks forwards, upwards, and outwards, and at its 

 upper end extends the whole length of the centrum ; the anterior faces of these basal 

 portions of the arch are concave and complete the atlantal cup, forming the upper 

 halves of its lateral borders. The neural surface of the atlas is nearly triangular, the 

 anterior angle being truncated by the upper border of the atlantal cup. The posterior 

 face is gently concave, with central dimple. It is wider than high and its upper 

 (neural) border is nearly straight, while its ventral border is truncated by the large 

 oblique facet for the second ventral wedge-bone (w.b. 2), which has a more extensive 

 union with the atlas than with the axis. The lateral faces of the atlas are convex 

 from above downwards, and gently concave from before backwards ; there is no trace 



Text-fig. 15. 



'- '■■'-^jL-ax.a 



wltis - Ml I "M 



od ~^ 2 - 



Atlas and axis of Pdmuustes philarclius, from left side. (R. 2438, § nat. size.) 



at.a., neural arch of atlas ; ax., axis ; ax.a., neural arch of axis ; n.sp., neural spine ; od., odontoid 

 (centrum of atlas) ; r.f., rib-facets on axis ; w.b. 1, w.b. 2, first and second subvertebral wedge-bones. 



of any facet for an atlantal rib. The neural arch of the atlas (at.a.) consists of two 

 separate halves, between the upper end of which there is a considerable interval. The 

 enlarged ventral portion unites with the centrum as above described, and forms the 

 upper lateral portion of the atlantal cup. The anterior border of the pedicle of the 

 arch is situated a little distance from the anterior border of the body ; on its outer 

 face is a roughened surface, apparently for the attachment of muscle or ligament. 

 Above, it expands into a smaller anterior lobe and a larger and more prominent posterior 

 one, the latter bearing on its inner face a facet, the posterior zygapophysis (p.z.), which 

 articulates with the pre-zygapophysis of the axis. The anterior lobe is roughened on 

 its inner face, and may have been united by ligament with the peculiar facet on the 



