0. G. Marsh — The Skull and Brain of Olaosaurus. 83 



Art. IX. — The Skull and Brain of Olaosaurus ; by 

 O. C. Marsh. (With Plates IV and V.) 



Iisr previous numbers of this Journal, the writer has de- 

 scribed and figured various remains of Cretaceous Dinosaurs 

 belonging to the genus Olaosaurus, and a restoration of one 

 very perfect specimen was given in the number for October 

 last." Another specimen apparently of the same species has 

 the skull in remarkable preservation, thus affording an oppor- 

 tunity to make out all its principal characters. This skull is 

 well represented in the accompanying plates, and the descrip- 

 tion is given below. The brain in this genus had many points 

 of interest, and a cast of the brain-cavity is also described 

 briefly and figured in the present communication. 



The Skull. 



The skull of Olaosaurus is long and narrow, with the facial 

 portion especially produced. The anterior part is only moder- 

 ately expanded transversely, Seen from the side (Plate IV, 

 figure 1), the skull shows a blunt, rugose muzzle, formed 

 above by the premaxillary and below by the predentary, both 

 probably covered in life with a thick, corneous integument. 



Behind the upper part of this muzzle is an enormous lateral 

 cavity, which includes the narial orifice, but was evidently 

 occupied in life mainly by a nasal gland, somewhat like that in 

 the existing Monitor, and also seen in some Birds. This cavity 

 is bounded externally by the nasal bone and the premaxillary. 

 The median septum between the two narial 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 subtriangular in outline. It is 

 formed above by the prefrontal, frontal, and postfrontal, and 

 below mainly by the jugal. There are no supra-orbital bones. 

 A distinct lachrymal forms a portion of the anterior border. 

 The infra-temporal 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 (Plate IV, figure 2), the skull of 

 Olaosaurus is subovate in outline, with the narrow portion 

 above. The premaxillaries and the predentary bone forming 

 the rugose muzzle are especially massive and prominent, and 

 the powerful lower jaws seem out of proportion to the more 

 delicate bones of the cranium. 



* This Journal, vol. xxxix, p. 423, May, 1890 ; vol. xliii, p. 453, May, 1892; 

 vol. xliv, p. 171, August, 1892, and p. 344, October, 1892. 



