AYilliston.] Mosasaurs. 133 



in section. Posteriorly on the outer side it has a free curved mar- 

 gin, inclosing a groove or fossa, which is narrowed and shallow 

 below, deeper and broader above, and which looks downwards, 

 outwards, and backwards. Its upper border is thinned, rough- 

 ened, and irregularly U-shaped. The anterior end of the bone 

 is irregularly roughened. 



The presplenial lies on the inner inferior side of the dentary, 

 extending as far forward as the fourth tooth. It appears on 

 the outer side at the inferior posterior part, where it is stout 

 and rounded on the lower margin. For a large part of its 

 extent it forms a deep groove or channel; that is, it is some- 

 what U-shaped in section, with the inner side longer and each 

 with a thin margin. At the posterior extremity the large 

 articular surface for union with the splenial is placed at right 

 angles to the long axis of the bone. It is rather deeply cupped, 

 oval or subcrescentic in outline, with the long diameter verti- 

 cal. Anteriorly the bone is thin, covering up the narrow, deep 

 groove for Meckel's cartilage. 



The <h ntary comprises more than one-half of the entire length 

 of the mandible. It is widened gradually behind, the upper 

 border straight or gently concave, the lower border somewhat 

 convex. Anteriorly it projects a short distance beyond the 

 first tooth, ending obtusely. On the inner side in this region 

 the groove for Meckel's cartilage begins a little distance before 

 the first tooth, the surface below flattened as far back as the 

 second for apposition with its mate. There are eleven teeth in 

 the dentary of this species, with eight or ten foramina in a row 

 below them exteriorly. Posteriorly the jaw projects a short 

 distance back of the last tooth, ending in a short, striated proc- 

 ess. From without, the jaw in front of the articulation is com- 

 posed almost wholly of the dentary, the presplenial showing 

 only at the posterior and lower part, the separating suture ap- 

 pearing nearly opposite the fourth tooth from the end and run- 

 ning back nearly parallel with the upper border. On the inner 

 side the presplenial reaches to in front of the middle of the 

 bone, opposite the fourth tooth. In front of this bone the groove 

 is partially closed by its thin margins. The articulation back of 



