1903.] Brown, A New Genus of Ground Sloth. 575 



On each side of this crest there is a shorter parallel ridge ex- 

 tending from above to about the middle of the skull. Im- 

 mediately above the left condyle there is a small foramen. 

 A deep groove separates the condyle from the occipital plate. 



Wedged in between the occipital and temporal regions is 

 the prominent mastoid, about half as wide as it is long, bounded 

 above by a continuation of the suture that separates the ex- 

 occipital from the supraoccipital. The outer margin is raised 

 into a prominent ridge forming the lower posterior boundary 

 of the temporal fossa. Immediately inside this ridge there is 

 a deep channel running upward, ending in the mastoid fora- 

 men. The entire posterior surface of the mastoid, between 

 the raised outer border and the exoccipitals, is a depressed 

 rugose area. The inner part of the lower end of the mastoid 

 is a cup-like depression for the articulation of the stylohyal, 

 which is not nearly so deep or so extensive as in Mylodon. 



The temporal fossa is uniformly smooth with an extensive 

 depression just above the mastoid, and a large protuberance 

 above this area, as in Mylodon. 



The zygomatic process of the temporal is a stout trihedral 

 bone, the upper edge of which is nearly straight; the external 

 surface is nearly flat and gently convex. The under surface 

 is broad and flat for the articulation of the lower jaw, without 

 any distinct glenoid cavity, thus allowing a great forward and 

 backward movement of the jaw. The anterior end of the 

 zygoma terminates in an obtuse point. 



The malar undoubtedly articulated loosely with the maxil- 

 lary and the zygoma, but unfortunately is missing on both 

 sides. 



The most prominent bone of the skull is the pterygoid, whicli 

 extends downward and outward in a broad plate ending in a 

 very wide convex border, more prominent than in any allied 

 genus. Posteriorly it descends from the sphenoid at an angle 

 of twenty degrees, forming the gently rounded distal end, 

 then sweeps upward, in a quarter arc of a circle. The external 

 surface is rugose with a prominent ridge parallel to the 

 zygoma dividing it into two depressions. In the posterior 

 part of the upper one is the large foramen ovale, which opens 



