PLIOSAUKUS. 11 



of the ridges may bifurcate and, in some of the largest teeth, the enamel is raised into 

 vermiculate sculpture. In some cases the smooth outer face may show traces of the 

 flattening which, in the later Kimmeridgian forms, is carried so far as to give the 

 crown of the teeth a more or less trihedral section*. Near the base of the crown, 

 on what is probably the posterior side, there is often a surface (PI. IT. figs. 2, lb, w.s.) 

 from which the enamel has been completely worn away. 



These teeth seem to be identical with those described by Sauvage (Bull. Soc. Geol. 

 France, [3] vol. i. (1873) p. 378) under the name Liopleurodon ferox, from the Oxford 

 Clay of Boulogne, and it was on this account that the name Pliosaurus ferox was 

 adopted for the species to which the remains here described belong. 



Vertebral Column (text-figs. 2-3). — No very well-preserved and complete vertebral 

 column is available for description ; that associated with the skull and mandible 

 (R. 3536) above referred to is the most nearly complete, but the vertebrae, particularly 

 in the dorsal and caudal regions, are much crushed and the neural arches are for the 

 most part lost. 



The atlas and axis have their centra closely fused together in the adult. The 

 centrum of the atlas (odontoid) forms the middle portion and the neural border of the 

 cup for the occipital condyle. The supero-lateral portions of the border of the cup 

 were evidently formed by the ventral ends of the neural arch, but this is not preserved 

 in any specimen. The ventral portion of the cup was formed by the anterior wedge- 

 bone. The centrum of the axis is oval in section, the long diameter being transverse. 

 Anteriorly it is closely united above with the odontoid, and below with the posterior face 

 of the hinder wedge-bone, which is interposed between the centra of the atlas and axis 

 ventrally. Judging from the structure of the atlas and axis in Peloneustes (see below, 

 p. 47, figs. 15, 16), the bases of the pedicles of the neural arch were very large, forming 

 the supero-lateral portions of the atlantal cup, as above noted, and extending down 

 to unite by a broad surface with the upper angles of the anterior wedge-bone, thus 

 excluding the odontoid from any share in the formation of the sides of the atlantal 

 cup, such as is seen in Murawosaurus (see Part I. pp. 92-93, text-fig. 49) and Crypto- 

 cleidus. In this manner of forming the atlantal cup, Pliosaurus and Peloneustes 

 resemble some Liassic Plesiosaursf and also the North American Cretaceous genus 

 Trinacromerum {Loliehorhyncops) described by Williston %. On the lateral surface 

 the united atlas and axis bear a large facet for a rib, the greater part of which is on 

 the axis, but apparently both the centrum of the atlas and the second ventral wedge- 



* See Owen's figure of tooth (Brit. Mus, 37408) of Pliosaurus maeromerus, Phillips (P. grand is, Owen), 

 in " Beptilia of the Kimmeridge Clay " (Mon. Pal. Soc. 1863), pt. ii. pi. xii. 



t Barrett, ''On the Atlas and Axis of a Plesiosaur," Ann. & Mag. Nat. Hist. [3] vol. ii. (1858) p. 361, 

 pi. xiii. 



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

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



c2 



