176 DINOSAURS OF NORTH AMERICA. 



to the long axis of the skull. It is formed almost wholly of the basi- 

 occipital, the exoccipitals entering but slightly or not at all into its 

 composition. The basioccipital processes are large and rugose. The 

 paroccipital processes are stout and somewhat expanded at their ex- 

 tremities, for union with the quadrates. 



The parietal bones are small and composed mainly of thearched proc- 

 esses which join the squamosals. There is no true pineal foramen, 

 but in the skull here figured (PI. XXV) there is the small unossified 

 tract mentioned above. In one specimen of Morosaurus a similar 

 opening has beeu observed, but in other Sauropoda the parietal bones, 

 even if thin, are complete. The suture between the parietals and 

 frontal bones is obliterated in the present skull, and the union is firm 

 in all the specimens observed. 



The frontal boues in Diplodocus are more expanded transversely than 

 in the other Sauropoda. They are thin along the median portion, but 

 quite thick over the orbits. 



The nasal bones are short and wide and the suture between them 

 and the frontals is distinct. They form the posterior boundary of the 

 large nasal opening, and also send forward a process to meet tbe 

 ascending branch of the maxillary, thus taking part in the lateral 

 border of the same aperture. 



The nasal opening is very large, subcordate in outline, and is par- 

 tially divided in front by slender posterior processes of the preinaxil- 

 laries. It is situated at the apex of the skull, between the orbits, and 

 very near the cavity for the olfactory lobes of the brain. 



The premaxillaries are narrow below, and with the ascending proc- 

 esses very slender and elongate. Along the median line these processes 

 form an obtuse ridge, and above they project into the nasal opening. 

 Each premaxillary contains four functional teeth. 



The maxillaries are very largely developed, more so than in most 

 other known reptiles. The dentigerous portion is very high and slopes 

 inward. The ascending process is very long, thin, and flattened, inclos- 

 ing near its base an oval foramen, and leaving a large unossified space 

 posteriorly. Above, it meets the nasal and prefrontal bones. Along 

 its inner border for nearly its whole length it unites with the ascending 

 process of the premaxillary. Each maxillary contains nine teeth, all 

 situated in the auterior part of the bone (PI. XXV, fig. 1 ). 



Along their upper margin, on the inner surface, the maxillaries send 

 off a thickened ridge, or process, which meets its fellow, thus excluding 

 the premaxillaries from the palate, as shown in fig. 27, opposite. Above 

 this, for a large part of their length, the ascending processes of the 

 maxillaries underlapthe ascending processes of the premaxillaries and 

 join each other on the median line. 



The orbits are situated posteriorly in the skull, being nearly over the 

 articulation of the lower jaw. They are of medium size, nearly circular 

 in outline, their plane looking outward and slightly backward. No 



