SCOTT : LITOPTERNA OF THE SANTA CRUZ BEDS. II5 



extended antero-posteriorly, forming a broad shelf. The glenoid cavity is 

 a large, almost circular depression, concave in both directions. The post- 

 glenoid process is rather short, but broad and heavy. 



The tympanic is very loosely attached to the skull and, in most cases, 

 has been lost ; it is very small and can hardly be said to form a bulla at 

 all, being little more than a flat, scale-like bone, with a large, irregular 

 auditory meatus, which is a mere hole and has no tubular prolongation. 

 The large periotic, which is concealed by the tympanic, is very dense 

 and antero-posteriorly elongated, laterally compressed, with convex 

 external, and nearly flat mesial surface. A small triangular area of the 

 mastoid portion is exposed on the surface of the cranium between the 

 exoccipital and squamosal. 



The jugal is long, slender and laterally compressed ; posteriorly, it 

 extends almost to the glenoid cavity, and anteriorly, it articulates with 

 the lachrymal, thus forming nearly the whole lower margin of the orbit. 

 As a whole, the zygomatic arch is thin and weak, but relatively consider- 

 ably longer than in Macraiichenia and the temporal opening is correspond- 

 ingly elongate, though actually quite small. It is true that Ameghino's 

 figure ('94«, 29, fig. 6) appears to show very long temporal openings, but, 

 in all probability, this is merely an appearance, due to the fact that the 

 matrix has not been removed from the root of the zygomatic processes. 

 As is shown in Plate XVII, fig. \b, the root of this process is much 

 extended antero-posteriorly, reducing the size of the temporal opening. 

 The lachrymal is rather small, and has but a moderate facial expansion ; 

 the foramen is single and is placed on the margin of the orbit, and the 

 lachrymal spine is a low tubercle. 



The frontals are very large, especially in transverse width, and form a 

 broad forehead, which, in undistorted specimens, is strongly convex trans- 

 versely and indicates the presence of large frontal sinuses. The postorbital 

 processes are long and slender, with rugose margins and so curved as to 

 be concave anteriorly and convex posteriorly ; they do not quite reach the 

 jugals and thus leave the orbits partly open behind. The supraorbital 

 margin is rough and tuberculated and is perforated by a large foramen, 

 which opens downward into the orbit, and, in addition, there is on each 

 side a large vascular opening, nearly midway between the sagittal suture 

 and the orbital margin. Anteriorly, the frontals send forward between 

 the nasals a pair of median processes, the size of which varies much in the 



