16 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 60 



a young Delphinapterus. The two plates of the pterygoid are sep- 

 arated from each other by a narrow interval anteriorly, but pos- 

 teriorly they are widely separated. Behind the middle the two 

 plates of the pterygoid are divergent, the external plate coming in 

 contact with the squamosal and parietal, and the internal plate over- 

 lapping the anterior margin of the basisphenoid behind and the 

 palatine in front. Anteriorly, the internal plate becomes somewhat 

 curved and contributes the lower outer surface for the nasal passage. 

 This portion of the internal plate bends inward and then outv»^ard 

 and is continuous anteriorl}'^ with the external plate of the pterygoid. 

 On the outside of the pterygoid and in front of the nasal passage 

 there is a lateral concavity. Below this concavity the lateral margin 

 of the pterygoid flares out. The external plate of the pterygoid ap- 

 parently contributes the horizontal backwardly projecting hamular 

 process which represents a posterior extension of the palatal sur- 

 face. The opening into the sinus between the tw^o plates of the 

 pterygoid lies internal and anterior to the falciform process of the 

 squamosal. Although this sinus is exposed along the right nasal 

 passage of this skull, it is because the horizontal hamular process of 

 the pterygoid broke off at that level. The anterior margin of the 

 external plate of the pterygoid is united by a S-shaped suture with 

 the palatine. The external plate of the pterj^goid articulates with 

 the squamosal, parietal, frontal, and palatine. The posterior half 

 of the external plate is arched over the alisphenoid, and excludes the 

 latter from the temporal fossa and from the outer wall of the 

 cranium. The upper and anterior portion of the internal plate of 

 the pterj^goid is applied to the ventral surface of the orbitosphenoid. 

 On the right side of the skull, the external plate of the pterygoid 

 is imperfect. One is thus permitted to study the relations of the 

 alisphenoid, internal plate of the pterygoid, and orbitosphenoid. 

 Fortunately these bones are essentially perfect in this skull. The 

 alisphenoid is broad, outwardly and upwardly curved, extending to 

 and suturally united above with the squamosal and parietal. Fur- 

 ther forward there is a small orbitosphenoid which projects laterally 

 on the ventral surface of the supraorbital process. Both plates of 

 the pterygoids are well preserved on the type skull, but on all the 

 others they have been destroyed in the region of the sphenoidal fis- 

 sure. By utilizing data obtained from all the specimens, it has been 

 possible to work out most of the details in this region. The condi- 

 tions observable in the region of the sphenoidal fissure appear to be 

 essentially the same as in the skull of an adult Delphina'pterus, al- 

 though in the young of the latter the anterior foramina are not so 

 well defined. As in Delphinapterus^ the orbitosphenoid forms the 

 lower portion of the anterior wall of the brain case. Between the 

 alisphenoid and the orbitosphenoid there is a sphenoidal fissure, 



