AVilliston.] Mosasaurs. 105 



in Mosasaurus. They articulate with the premaxilla to a very 

 slight extent only at the tip. The suture between them and the 

 palatines is indistinguishable. 



In Platecarpus the vomers resemble those in the two preceding 

 genera. A view of the anterior outer side is shown in pi. xix, 

 f. 4, giving the articulation with the maxilla. In none of the 

 three genera is there any indication of the long, tongue-like 

 process of the premaxillary, as described below, and no distinct 

 surface for union with the premaxilla. The emargination back 

 of the articular face is longer and deeper in the horizontal max- 

 illary plate of this genus. 



The vomer in Tylosaurus is very much elongated, as in Mosa- 

 saurus. Anteriorly it ends in a thin vertical plate lying contig- 

 uous with its mate and inserted between the two plates of the 

 premaxilla on the under side, as described for that bone. On 

 the outer side, as far back as the middle of the second maxillary 

 tooth, it has a shallow longitudinal groove for articulation with 

 the thin, vertical, tongue-like plate of the premaxilla. The ar- 

 ticulation with the maxilla extends back of this as far as the 

 middle of the fourth maxillary tooth, presenting an elongated 

 sutural surface. Posterior to this articulation the bone is con- 

 stricted as in the other genera to form the anterior palatine 

 foramina, which lie below the anterior end of the nares, and 

 thence gradually widens, standing nearly vertically. The union 

 with the palatines is so close that it cannot be distinguished, 

 the bones continuing in the same line and in apposition nearly 

 as far back at the anterior end of the dilated portion of the pala- 

 tines. In the posterior part the bones slope outward from the 

 middle line, where they are in close contact, inclosing a long, 

 slender, oval opening between them and the maxillae, in large 

 part below the external narial opening. Just back of the max- 

 illary articulation the bones diverge a little for a short distance 

 to show the inferior border of the prem axillary, and the under 

 surface here shows an oblique groove running backwards and 

 outwards, as though for the passage of a nerve or blood-vessel. 



