36 MAEINE EEPTILES OF THE OXFORD CLAY. 



Dentition (PI. I. figs. 1-8). — The dentition both in the upper and lower jaw seems 

 to be undergoing reduction. This is shown by the relatively small size of the teeth, 

 and, in the adult, by their apparent absence from the hinder part of the jaws, where 

 no traces of alveolar divisions can be seen in the dental groove. In the young the 

 front of the jaw bears numerous closely-set teeth, in one case as many as eleven in a 

 space of 7 cm. (PL I. figs 7 and 8). In this case the teeth are in actual contact with 

 one another, and are inclined a little backwards. In older jaws the teeth in nearly all 

 cases have fallen out, their former position in the wide dental groove being marked by 

 a series of alveolar pits, which are separated from one another by transverse rounded 

 ridges. The latter are verj^ distinctly marked on the front of the jaws, but become 

 fainter and fainter as they are followed backwards, till they finally disappear altogether. 

 Judging from the relative smallness of the teeth and the large size of the alveolar pits, 

 it would appear that the teeth in the adult must have been very loosely implanted in 

 a soft gum, the putrefaction of which after death would account for the detached 

 condition in which the teeth are ordinarily found. In some cases, as in other Ichthyo- 

 sauria, the roots of the teeth are marked by a deep pit resulting from the absorption 

 of their substance before the advancing point of a replacing tooth. 



The teeth themselves vary considerably in form, some being much curved, others 

 nearly straight. The crowns (PI. I. figs. 2 and 4-6) are sharply-pointed cones; they 

 are usually curved and nearly circular in section ; they are covered with a fairly thick 

 coat of enamel, which in the middle portion of the crown is raised into longitudinal 

 ridges which disappear towards the tip and the root. Beneath the enamel-clad crown 

 the tooth thickens considerably, and the smooth dentine is exposed or at most covered 

 with a very thin layer of cement (PI. I. fig. 3) ; in this region there are sometimes 

 rounded ridges running round the tooth. Lower down the dentine is thickly covered 

 with cement, and is thrown into strong longitudinal folds; in some cases (PL I. fig. 1) 

 these are so deep that their inner ends meet and enclose a small separate portion of the 

 pulp-cavity. This cavity is very small in the crown, but lower down widens out and 

 near the base is filled with a loose network of cement (see PL I. figs. 1 and 3). 



The hyoid arch is represented only by a pair of elements (the ceratohyals ]). They 

 are curved and flattened bars of bone, widening out gently towards their extremities, 

 which are abruptly truncated at right angles by surfaces that appear to have been 

 tipped with cartilage during life. Towards the ends of the bone the walls are 

 frequently crushed in, as if the interior had been hollow or at least fiUed with 

 comparatively soft tissue. 



Vertebral Column (text-figs. 24-31). — In all the specimens hitherto examined the 

 centra of the axis and atlas vertebrae are akeady fused together, though in some cases 

 the line of junction is still plainly visible (text-fig. 24, D). The anterior face of the 

 atlas is deeply cupped and is roughly triangular in outline, the angles being rounded 

 off. The ventral angle is farther truncated by a roughened surface [w.b.f.) looking 



