MUR^NOSAUECrS. 93 



not meet in the middle line, the floor of the neural canal being formed by the odontoid ; 

 and they only form a very small share of the supra-lateral borders of the atlantal cup, 

 being widely separated from the ventral wedge-bone (text-fig. 49, B). In some Liassic 

 Plesiosaurs, on the other hand, the odontoid takes a much smaller share in the 

 formation of the atlantal cup, while the lateral pieces extend downwards and unite 

 extensively Avith the upper end of the wedge-bone *, so that the odontoid only forms 

 the middle portion and the neural border of the cup. Curiously enough, this condition 

 is repeated in the American Cretaceous Plesiosaur Trinacromerum (BoUchorhynchops) 

 osborni, described by Williston f. In fact, in the structure of the articulation with 

 the skull, Murcenosaurus seems to retain a much more primitive condition than is 

 found in some, at least, of the Liassic and Cretaceous types. 



The upper portions of the lateral (neural) pieces of the atlas run backwards and bear 

 at their extremities comparatively large zygapophysial facets, by which they uuite 

 with the corresponding surfaces on the side of the arch of the axis (ax. a.). The two 

 halves of the arch are separated from one another superiorly by a considerable interval. 

 The anterior sub vertebral wedge-bone (a.w.b.), as already noted, forms the lower 

 portion of the atlantal cup. Above it unites with the odontoid in a suture running 

 backwards and a little downwards. Posteriorly it is bounded by a nearly vertical 

 suture, but whether this separates it from the second subvertebral wedge-bone (? p.w.b.) 

 or from a ventral prolongation of the odontoid, cannot be determined ; its free 

 ventral surface is raised into a strong hypapophysial ridge (hi/.r.), which is continued 

 back on the ventral surface of the bone behind, and dies away at the middle of the 

 axial centrum (ax.c). This latter is similar in every respect to the succeeding 

 centra, except that it is fused with the odontoid in front. Above it bears a neural 

 arch (ax.a.) with a spine, the highest point of which is at its posterior end. The 

 pedicles of the arch widen out considerably at their ventral end, so that the surface 

 for union with the centrum projects anteriorly a little beyond the axial centrum and 

 unites also with the odontoid. The anterior and posterior zygapophyses are both 

 well developed. Beneath the surface for union with the neural arch the sides of the 

 centrum are a little concave, but then turn outwards and bear the facet (r.-) for the 

 large axial rib ; the extreme anterior edge of this facet seems to be borne by the 

 odontoid, Avhich also has a small prominence apparently indicating the presence of a 

 very rudimentary atlantal rib {r}). The posterior face of the axial centrum is gently 

 concave and is a rounded oval, the position of the neural canal being marked by a 

 slight concavity. 



The above description of the atlas and axis is founded on a specimen belonging to a 



* See Barrett, " On the Atlas and Axis of a Plesiosaur," Ann. Mag. Nat. Hist. [3] vol. ii. (1858) p. 361, 

 pi. xiii. 



t "North American Plesiosaurs, Pt. I.," Field Columbian Museum, Geological Series, vol. ii. no. 1 (1903) 

 p. 32, pi. xxii. fig. 5. 



