Wedge Bones in the Plesiosaurus. 221 



like the first wedge-bone in the Plesiosaurus, and forms the lower 

 third part of the cup for the occipital condyle : it articulates be- 



h Cervical vertebra, Amblyrhynchns. 



hind to a second similarly-shaped wedge-bone {ex, ex) ; above to 

 the part c a, (indicated by the dotted outline on the neural arch 

 of the atlas which covers it,) which answers to the body of the atlas, 

 or rather the central part of the body of the atlas, in the Plesiosaurus 

 (c a, fig. 3) ; but which there also immediately supports the neural 

 arch (n a), whilst in the recent Saurian the base of each neura- 

 pophysis (na, fig. 4) descends to rest upon the angles of the 

 base of the * wedge-bone i (ca, e x) which represents the inferior 

 peripheral part of the body of the atlas. The interspace between 

 the bases of the neurapophyses is occupied by the distinct ossicle 

 (c a) which adheres closely by a flat surface to the body of the 

 axis (ex), and forms the bottom of the articular concavity for the 

 occipital condyle : it is the part described by Cuvier as ' piece 

 analogue a Fodontoide ' in the Monitor*, and it is plainly the 

 homologue of the part of the body of the atlas which is joined 

 by a flat surface, and early anchylosed, to the body of the axis in 

 the Plesiosaurus and Ichthyosaurus. In the existing Lacertians 

 the 'odontoide' differs, however, from the anchylosed atlas of 

 the Enaliosauria by taking no share in the support of the neura- 

 pophyses of the atlas : the ossification of these has obviously ex- 

 tended deeper into the sides of the notochordal capsule, so that 

 they articulate directly with the wedge-bone (c a, e x) developed in 

 the inferior part of that capsule : and the central ossification (c a) 

 adapted to the lower half of such atlantal vertebral ring is pro- 

 portionally reduced. The first ( wedge-bone ' or cortical part of 

 the body of the atlas (fig. 4, c a, e x) is carinate below in the 

 Amblyrhynchus, and the keel is produced backwards into a short 

 spine. The second ' wedge-bone/ ex, ex, is similarly shaped, 

 but rather larger than the first. Its base articulates behind with 

 the body of the axis, c x, above with the odontoid piece, c a, and 

 * Ossem. Foss. v. pt. 2. p. 283. 



