168 MAEINE EEPTILES OF THE OXFOED CLAY. 



the posterior side there is a main ridge, more prominent than the rest and continuous 

 almost to the top of the crown, forming in some cases a very slightly marked keel. 

 The rest of the surface (PL III. fig. 4 a) is covered with fine ridges of varying length, 

 running in a generally longitudinal direction and often anastomosing. The crown is 

 circular or nearly circular in section throughout its length ; in Cryptocleidus the crown 

 is sometimes slightly compressed, so that in section it is oval. 



The roots of the teeth in both genera are very long and circular in section ; their 

 surface is smooth and the pulp-cavity is large. 



Vertebral Column. — The atlas and axis (text-fig. 78) are very similar in structure to 

 those of Micrcenosaurus, but differ in some details. As in the case of the other cervicals, 

 the centra of these vertebrae, particularly that of the axis, are shorter than in Murceno- 

 saurus. In the formation of the cup for the occipital condyle the bases of the 

 lateral pieces of the neural arch of the atlas (at. a.) take a somewhat greater share than 

 in the other genus, but they are still separated by a considerable interval from the sub- 

 vertebral wedge-bone (a.w.b.) which constitutes the lower fourth of the cup. As in 

 Murcenosaurus, the lateral pieces of the neural arch do not unite above, but run back 

 and articulate with the anterior zygapophyses of the axis ; from the presence of a pair 

 of peculiar facets on the exoccipitals in the young specimen (PI. IX. fig. 4 «,/.), it 

 seems probable that a pro-atlas was present, but whether single or paired there is no 

 means of ascertaining. In the axis the neural arch [ax.a.) is lower than in Murceno- 

 saurus, and there is not so well-developed a neural spine ; on either side of the base 

 of the neural spine there is a strong ridge, not present in the other genus. The 

 posterior zygapophyse.*! (i^.z.) are very large, and project a long way behind the level of 

 the posterior face of the centrum ; their facets are flat and look outwards and down- 

 wards. The posterior face of the centrum is concave in the middle with a raised and 

 convex outer portion; it is considerably wider than high. The facet (r.y.) for the rib 

 of the atlas is much larger than in llurcenosaurus, and its antero-inferior portion is 

 borne by the subvertebral wedge-bone. The rib of the axis (r.-) is larger than that of 

 the atlas ; the facets for these two ribs are confluent. The subvertebral wedge-bone 

 {a.w.b.) bears a strongly developed hypapophysial ridge (Jiy.r.), which forms a distinct 

 anterior prominence not seen in Muramosaurus ; the ridge only extends backwards on 

 to the anterior portion of the centrum of the axis. In the young specimen (text- 

 fig. 78, A, B) in which the constituent elements of the atlas-axis are still separable, 

 there is no trace of the presence of a second subvertebral wedge-bone. In addition 

 to the atlas and axis there are thirty other cervical vertebrse. Of these the centra 

 are much shorter than in Ifura^nosaurns, and their articular ends are strongly concave 

 in the middle with rounded margins; in outline they are transversely oval, the upper 

 border beneath the neural canal beins; a little concave. The ventral face of the centra 

 IS gently concave from before backwards and also from side to side, with the exception 

 of the longitudinal ridge between the pair of nutritive foramina. Above the rib- 



