220 DINOSAURS OF NORTH AMERICA. 



and seen also in some birds. This cavity is bounded externally by the 

 nasal bone and the premaxillary. The median septum between the two 

 uarial orifices was only in part ossified, the large oval opening now 

 present in the skull probably having been closed in life by cartilage. 



The orbit is very large and subtriaugular in outline. It is formed 

 above by the prefrontal, frontal, and postfroutal, and below mainly by 

 the jugal. There are no supraorbital bones. A distinct lachrymal 

 forms a portion of the anterior border. The infratemporal fossa is 

 large, and is bounded above by the postfrontal and squamosal, and 

 below by the jugal. The quadrate forms a small portion of the posterior 

 border. 



Seen from in front (PI. LXXII, fig. 2), the skull of Claosaurus is sub- 

 ovate in outline, with the narrow portion above. The premaxillaries 

 and the predentary bone forming the rugose muzzle are especially mas- 

 sive and prominent, and the powerful lower jaws seem out of propor- 

 tion to the more delicate bones of the cranium. 



Seen from above (PI. LXXII, fig. 3), the structure of the skull itself 

 is shown to the best advantage. In front are the large premaxillaries, 

 deeply excavated for the uasal openings. These bones are separate, 

 and each sends back a long, slender process inside the anterior pro- 

 jection of the nasal, and a still longer process forming the lower border 

 of the narial orifice, anu extending to the lachrymal. The front of the 

 premaxillaries is especially massive, and its surface rugose, indicating 

 that it had been covered with a horny beak. The lower border is sharp, 

 conforming to the corresponding surface of the predentary bone, which 

 was doubtless also inclosed in a horny covering. The premaxillaries 

 were entirely without teeth. 



The nasal bones are long and slender, and especially produced in 

 front, where they embrace the posterior median extensions of the pre- 

 maxillaries. They also meet the lateral processes of the premaxillaries 

 behind the nasal openings, and likewise touch the lachrymals. Farther 

 back they meet the prefrontals and closely unite with the froutals, as 

 shown in IT. LXXII. 



The frontal bones are quite short, and nearly as wide as long. They 

 are united to each other by a well-marked suture. Their upper surface 

 is smooth, and there is a slight depression on either side, posterior to 

 the suture with the prefrontals. Each frontal bone forms a portion of 

 the upper border of the orbit, and behind this meets the postfrontal. 

 Posteriorly the froutals form the anterior border of the supratemporal 

 fossa 3 , and between these unite by suture with the coossified parietals. 



The latter bones are quite small, and appear on the upper surface of 

 the skull mainly as a narrow ridge separating the supratemporal fossa 1 , 

 and ending behind in a point, between the median processes of the 

 squamosals. The parietals expand below, where they cover the pos- 

 terior portion of the brain cavity. 



The squamosal bones are robust, and their position and connections 



