FAUNA OF THE GOSAU FOKMATION. 



025 



posterior end to the top of the coronoid process, which is imperfectly 

 preserved, is 3| centim. The external surface is smooth, rounded 

 at the base, with a strong rounded ridge descending from the ante- 

 rior margin of the coronoid process, and extending downward and 

 forward along the jaw, dying away in front, and placed well above 

 the middle of the side. Above this ridge the area of the jaw, ex- 

 tending inward to the alveoli, is flattened almost horizontally at 

 the back ; but the area becomes more oblique anteriorly, and undi- 

 stinguishable as the ridge subsides. Along its upper and outer 

 margin towards this ridge is a series of foramina which are elongated 

 or ovate — four of them larger, and half-a-dozen smaller. The area 

 below the ridge is flattened towards the posterior limit of the dentary 

 bone ; anteriorly it is flattened and pointed, being bevelled on the 

 inferior border for its union with the other ramus, and on the supe- 

 rior border terminating in an oblique area, which is compressed from 

 side to side and channelled by a somewhat deep groove. Whether 

 this groove is merely vascular, or whether it may have contained a 

 few premaxillary teeth, is a matter upon which I have no evidence. 

 It is about 16 millim. long, wider and deeper behind than in front, 

 and, as in the Wealden Iguanodon, has the inner border more ele- 

 vated than the outer border. Below it are three or four vascular 

 foramina. Both the inner and outer extremities of the jaw below 

 are roughened, and indicate that the symphysis was loose, but held 

 together by ligamentous union. As usual, the external outline of 

 the bone, when viewed from above, is moderately convex, and its 

 thickness from within outward continues to increase from before 

 backward almost to the coronoid process. The inner side, which is 

 slightly crushed, displays ten alveoli. Portions of five or more teeth 

 are seen in the jaw; and there are impressions of others and empty 

 sockets, indicating ten in all. The first tooth, which is unfortu- 

 nately imperfect (wanting the extremity of the crown), is remarkable 

 for the smoothness of its inner surface, which, however, is elevated 

 into a very strong median ridge, leaving the sides slightly concave. 

 The serrations visible on the anterior margin are slight, and do not 

 extend down the tooth. It is not sufficiently elevated to have come 

 into wear. All the succeeding sockets are empty, owing to the teeth 

 having dropped out; but most of them show successional teeth 

 coming up, which have not yet reached the level of the outer alveolar 

 margiu. The second and third teeth are broken away on their ex- 

 ternal part, and not recognizable. The fourth tooth, also wanting 

 the extremity of the crown, still shows the same enormously deve- 

 loped median ridge ; but external to it are, on each side, about half- 

 a-dozen fine parallel ridges which have sulcations behind them of 

 about their own width. The fifth tooth only just shows the top of 

 its crown coming up low down in the alveolus. The sixth tooth^is 

 the best developed, was apparently the largest, and occurs near the 

 thickest part of the jaw. Its pattern is like that of the last described ; 

 only the strong median ridge or keel is much sharper, and the lateral 

 concavities deeper, in accordance with the width of the tooth. The 

 lateral ridges run up and terminate in the sharp rounded lanceolate 

 Q.J.G.S. No. 148. 2t 



