AVilliston.] Mosasaurs. 101 



is complete in all the known forms, forming the posterior and 

 inferior border of the orbit. Below, on the inner side, it is 

 suturally attached to the transverse bone, which does not reach 

 forward to the maxilla, or so only to a very slight extent. The 

 parietal sends down on each side a flattened, w^ing-like process 

 for union with the petrosals and supraoccipital, bounding the 

 brain cavity externally in part. 



The posterior aspect of the skull presents an elongated open- 

 ing below the parietal arch, bounded below by the conjoined 

 exoccipital and paroccipital and above by the parieto-squamosal 

 arch. The teeth exist in a single row on the maxillae, mandi- 

 bles, and pterygoids, and in a double row of four on the con- 

 joined premaxillary. The vomers are elongated bones of the 

 palate attached anteriorly to the premaxillary and maxillary, 

 posteriorly to the anterior elongation of the palatines. The 

 crowns of the teeth are simple, conical, and recurved, sometimes 

 nearly round in cross-section, at other times flattened oval with 

 a posterior and anterior cutting carina, the surface elsewhere 

 smooth, or narrowly faceted and striated. The crown is at- 

 tached to a spheroidal mass of ostein, which is not, however, a 

 true root, and which projects beyond the margin of the bones 

 in which they are inserted in a cavity. This base is an ossifi- 

 cation of the tissue surrounding the blood-vessels and nerves, 

 and is frequently dislodged entire from the jaw. On the inner 

 side posteriorly of this base there is usually a smaller excava- 

 tion, in which the young tooth may be found. Rarely is a jaw 

 seen in which all the teeth are complete. Some will be lost, 

 showing only the empty cavity of the socket, while others will 

 be found in different stages of growth. The crown is covered 

 with enamel. The teeth of the pterygoids are smaller, some- 

 times much smaller, than those of the jaws, and are usually more 

 curved. 



The posterior flattened portion of the palatines articulate 

 closely with the posterior end of the maxillae on the outer side, 

 the anterior thinned portion of the pterygoids on the inner, the 

 vomers anteriorly, and the descending process of the prefrontal 

 on the .superior side posteriorly. Between the anterior process 



