PLTOSAUBUS. 9 



the maxilla by a curved suture, and in contact above for a short distance with the 

 prefrontal ; this element is probably the lachrymal (I.). It is produced inwards into a 

 broad ridge, which forms the anterior wall of the orbit, and curving upwards and 

 backwards becomes continuous with the rounded crest on the lower surface of the 

 cranial bones which marks the inner limit of the orbit. The external nares (nar.) are 

 oval apertures about 5 cm. long, situated about 57 cm. from the tip of the snout, that 

 is to say some distance behind the internal nares. The distance between them is about 

 9"5 cm., probably exaggerated by the crushing the skull has undergone. As above 

 described, the posterior outer and half the inner border are formed by the maxilla, the 

 remainder being enclosed by the bone here regarded as the nasal. 



In Pliosaurus a well-developed sclerotic ring seems to have been present, though 

 so far it is only known from a few plates. Each plate consists of fibrous-looking 

 bone ; the outer and inner borders are concentric and the outer the longer, while 

 the lateral edges are nearly straight and are grooved or tongued, as the case may be, 

 for union with the adjacent plates. The outer and inner borders are thin and sharp 

 and are irregularly serrated ; the bone thickens regularly to a line a little nearer to 

 the inner than to the outer border. These plates differ from those of Ophtlialmosaurus 

 in being only gently convex externally and concave internally, and not curving round 

 on the back of the eye-ball. 



Mandible (PL II. figs. 1, 1 a). — The mandible is distinguished from that of Pelo- 

 neustes in having a somewhat shorter symphysis (sym.), which, at the same time, is 

 longer than that found in Simolestes. The symphysial region extends back to the 

 seventh tooth, and between the third and fifth teeth it widens considerably ; it is 

 formed almost entirely by the dentaries, the splenials only entering into it for a short 

 distance on the ventral side. The upper surface of the symphysis between the rows 

 of alveoli is raised into a prominent ridge, on either side of which is a series of 

 depressions probably marking the places in which the replacing teeth developed ; 

 posteriorly the ridge divides, passing into the raised inner alveolar borders of the rami. 

 These diverge from one another at a very acute angle ; at first they are straight, then 

 curve gently inwards to the articular surfaces for the quadrates. The anterior part 

 of each ramus, as far back as the coronoid angle, is formed externally by the large 

 dentary, the inner face of which is covered by the closely adherent coronoid (spl. in 

 fig. 1) and splenial, the latter of which roofs in the dental canal. 



The coronoid {spl. in PI. II. fig. 1) is a very long thin strip of bone very closely 

 adherent to the inner face of the dentary (d.). At its hinder end it forms the inner 

 face of the coronoid angle, immediately beneath which it unites in complex suture with 

 the surangular (art. & surang.). Its upper border is parallel with and close to the 

 edge of the dentary, which forms the inner side of the groove within the line of 

 alveoli ; below it is overlapped by the splenial. Its anterior end extends to, or 

 perhaps even enters, the symphysis. 



PART II. C 



