6 MARINE EEPTILES OF THE OXFORD CLAT. 



a hemisphere. The condyle is usually about equally convex in all directions, but 

 sometimes {e. g. in type specimen, PI. I. figs. 13, 14) it may be somewhat pinched in 

 laterally towards its upper end ; the outline is nearly circular, but it is flattened above 

 for a short distance [n.c), where it forms the lower border of the foramen magnum; 

 its surface is usually marked by a series of slight concentric ridges, and there is near its 

 middle a small pit or dimple, probably marking the original position of the notochord. 

 The upper surface of the bone (text-fig. 1, A) is occupied in the middle line by a smooth, 

 slightly concave surface {n.c), extending from the upper border of the condyle to the 

 anterior edge ; this surface, which is the floor of the neural canal, is narrowed somewhat 

 in the middle by the encroachment of the large roughened concave surfaces for union 

 with the exoccipitals [exo.f.). In front of, and a little to the outer side of these surfaces 

 there is in many specimens a slight prominence terminating in a smooth facet (op.f.), 

 which appears to have supported the anterior portion of the opisthotic. On the sides 

 of the bone in front of the condyle there is a smooth area slightly concave from before 

 backwards, and in fi-ont of this a broad roughened surface (st.f.) looking outwards 

 and a little downwards, with which the head of the stapes articulates. The ventral 

 surface is also occupied by a smooth area which ends in front in a straight or slightly 

 concave border, along which the bone is in contact with the basisphenoid. The 

 anterior face slopes somewhat backwards and is entirely occupied by a coarsely 

 roughened surface for cartilage, usually divided into two bosses by a slight median 

 groove ; it is clear that even in old individuals the basioccipital and basisphenoid were 

 only in contact at most along their ventral edge, and were separated above by a thick 

 wedge of cartilage (text-fig. 1, C). 



The exoccifdtals {exo., text-fig. 2, A & B, also text-fig. 4) are short, stout, 

 columnar bones which form the lower part of the lateral border of the foramen 

 magmmi. At their ventral end [hoc.f.) they are considerably expanded, their base 

 extending forwards in a long tongue-like process, and in consequence of this 

 their surface for union with the basioccipital is very extensive. Their flattened 

 posterior face seems to have sloped somewhat forwards ; near the middle of its outer 

 border it is perforated by a large foramen (XIl'), the inner opening of which lies at 

 about the middle of the inner (cranial) surface. This is concave from above down- 

 wards, and in addition to the large foramen just referred to, there is an oblique slit-like 

 opening (XII) just anterior to it. The anterior border is strongly notched, the notch 

 apparently forming the posterior border of the so-called foramen jugularis {J for.). 

 Judging from the arrangement of the nerve-exits of this part of the skull of Hatteria, 

 as described by Osawa*, it seems probable that the two foramina (XII, XII') perforating 

 the exoccipital, transmitted two branches of the XII nerve, of which the posterior is the 

 larger, and that the IX-XI nen"es passed out through the jugular foramen, the hinder 

 border of which is formed by the exoccipital as above described. The outer face of the 

 * Archiv f. mikroscop. Anatomie, vol. li. (1897) pp. 49-4-5. 



