42 MARINE EEPTILES OF THE OXFORD CLAY. 



postfrontal by a short interval ; its outer edge forms the border of the orbit for some 

 distance, and on its lower face is part of the ridge which is mentioned above as to some 

 extent separating the orbit from the nasal cavity. Its relations to the other bones at 

 its anterior end are not clear ; it seems, however, to have extended nearly, if not quite, 

 to the narial opening. On its outer side in front it joins a small bone which seems 

 to have formed the anterior lower rim of the orbit and is probably the lachrymal. 

 Ventrally this element unites with the upper edge of the maxilla. The narial 

 aperture (nar.), as already noted, has its inner border apparently formed by a 

 prolongation of the frontal ; its anterior margin forms a notch in the hinder edge of 

 the maxilla. There is some indication that the posterior and outer borders are 

 formed by a small distinct element, which, if actually present, must be regarded as a 

 nasal. 



The premaxilla? (pmx., PL IV. figs. 1 , 2) are separated from one another throughout 

 their length by a distinct suture. Each consists of an enlarged anterior portion 

 forming the anterior end of the snout, which bears teeth, and an elongated narrow 

 facial process which extends back to the level of the anterior border of the orbits and 

 joins the parietals on the outer surface of the skull and the frontals below. Each 

 premaxilla bears six tooth-sockets, of which the first is the smallest and is directed 

 downwards and forwards, the second is larger, and the third and fourth the largest of 

 the series ; the fifth and sixth decrease in size successively. Behind the last there is 

 a short diastema crossed by the maxillo-premaxillary suture. On the palate the 

 extension of the premaxilla is very small ; the suture with the maxilla after crossing 

 the alveolar border runs a little backwards (about 2 cm.), there forming the outer 

 border of a short palatal process, the inner side of which unites with the vomers, which 

 run forwards between the preraaxilhe to a point opposite the interval between the 

 fourth and fifth teeth. In front of this the prem axillae meet in the middle line of the 

 palate, which is here raised into a low platform separated from the alveoli by a deep 

 groove. The upper surface of the anterior portion of the united premaxillae is evenly 

 convex from side to side, and the bone is marked by a number of small foramina ; this 

 region is widest at about the level of the fourth tooth, in front of which it narrows to 

 the rounded tip of the snout, while posteriorly it also narrows slightly to the level of 

 the diastema. The maxillo-premaxillary suture on the facial surface curves upwards 

 and backwards till it is about To cm. from the middle line, then runs straight back 

 parallel to the long axis of the skull, forming the outer edge of the facial processes, 

 which remain of the same width till close to their hinder end. At the external nares 

 or a little in front of them, the premaxilla? pass between the frontals and at their 

 posterior extremity are just in contact with the overlapping parietals. Throughout 

 their length the facial processes are strongly convex and together seem to have formed 

 a rounded ridge along the middle of the preorbital region of the skull. 



The maxilla (mx., PI. IV.) is a very large bone extending back to the level of the 



