90 MARINE REPTILES OF THE OXEORD CLAY. 



angle, at the bottom of which opens the median eustachian cana] (eu.m.) ; the 

 anterior border of this seems to he formed by the basisphenoid, although in one 

 case observed it appears that the opening may be completely surrounded by the 

 basioccipital. Almost immediately within this opening are the orifices of two lateral 

 canals which run outwards and slightly upwards between the basioccipital and the 

 basisphenoid, narrowing in the middle and opening laterally into the lateral eustachian 

 openings (eu.L), which appear to lie in the depression at the point where the sutures 

 between the basioccipital, exoccipital, and basisphenoid meet; the posterior pro- 

 longation of the pterygoid also appears to help in forming the anterior border of the 

 opening (text-fig. 33). In some cases there is a groove running from the lateral 

 eustachian openings down the sides of the basis cranii towards the median opening, 

 but in the present genus this character is not very well marked. In addition to the 

 lateral eustachian opening, just within the median aperture there seems to be a median 

 canal running forwards and upwards into the body of the basisphenoid, dividing above 

 into two lateral branches ; there is also a small, probably vascular, foramen running 

 back into the body of the basioccipital. 



The position of the foramina for nerves and blood-vessels on the occipital surface 

 (PI. V. fig. 3) is not easy to determine with certainty. On either side of the foramen 

 magnum is a pair of small openings which probably transmitted the hypoglossal nerve. 

 Below these and in the line of the suture between the exoccipital and basioccipital, 

 there is another foramen, with in some cases one or more small apertures near it ; this 

 opening is regarded by Auer * as transmitting the vagus group of nerves, but it seems in 

 all cases observed by me to be too small for this, and more probably received some small 

 blood-vessel. If this is so, then it would appear that there is no separate external 

 opening for the vagus, the foramen for which is confluent with the carotid foramen 

 situated in the notch between the outer face of the ventro-lateral process of the 

 exoccipital and its prolongation on to the quadrate. Above and external to this there 

 is another opening on the paroccipital process, probably for the facial (vn) nerve. 

 In a skull of Mystriosaurus (B.M. no. 33095) the vagus openiug is distinct from that 

 for the carotid, and the latter is not in the angle as just described in Steneosaurus, but 

 on the occipital surface and in the same plane as the hypoglossal and vagus foramina. 



Mandible (PI. V. fig. 4). — The mandible is chiefly remarkable for the great length 

 of the symphysis, which is formed mainly by the dentaries, but posteriorly also to a 

 varying extent by the splenials ; an anterior prolongation of the coronoid also extends 

 on to the dorsal face of the symphysis (PI. V. fig. 4). The relative length of the 

 symphysis and the number of teeth carried by it vary much in the different species, 

 affording an important means of identification. The bones forming the mandible 

 are the six pairs usual in the group, viz., dentaries, splenials, coronoids, angulars, 

 surangulars, and articular?. 



The dentaries \d.) are the largest elements: for the greater part of their length they 



* Palceontograpliica, vol. lv. (1909) p. 273. 



