146 University of California Publications in Geology [Vol. 10 



extend backward in the root of the pterygoid (PI. Ill), and branch 

 off from there forward into the alveolar wall of the maxilla." 2 '' An 

 extensive inflation of the pterygoid in skulls of the tree sloths 

 Choloepus and Hemibradypus has been noted by Anthony. 20 With 

 reference to the air sinuses in the skull of the former genus, he states : 



Le sinus sphenoidal s'etend lateralement a l'interieur des pterygoi'des, se 

 prolonge dans le temporal, envahissant la base de l'apophyse zygomatique et 

 venant presque au contact des prolongements lateraux du sinus frontal. Le 

 crane osseux presente un orifice situe a la face interne et anterieure de chaque 

 pterygoi'de et qui conduit dans ce sinus.- 7 



Nothrotherium therefore resembles Choloepus very closely not only 

 in the large size of the pterygoid inflation but also in the presence of 

 an orifice perforating the inner wall of the pterygoid. 



The alisphenoid forms not only the roof of the pterygoid bulla but 

 the dorso-internal wall as well. In some skulls an internal continua- 

 tion of the vacuity of the pterygoid extends toward the middle line. 

 In skull 634 this extension, lying below the ventral surface of the 

 basisphenoid, is separated from its fellow of the opposite side by a 

 median partition only 2 mm. in thickness. The development of these 

 internal sinuses varies from the extreme case seen in no. 634 to their 

 entire absence as in skull 15. 



The basisphenoid is concave transversely and tapers towards the 

 anterior end where the pterygoid bullae approach each other most 

 closely. 



The basioecipital in most skulls is short, relatively much more so 

 than in Megalonyx. The tympanic, as seen in no. 208, resembles 

 Rapalops in the incomplete, irregular circle of bone which encloses 

 ventrally the large external auditory meatus. Anteriorly the tympanic 

 may touch the dilated wall of the pterygoid. Posterior to the 

 tympanic is the stylo-hyal process with its ovoid depression. It is 

 situated much closer to the occipital condyle than in Megalonyx. 

 The small condylar foramen lies in a pit and may be entirely hidden 

 by the border of the occipital condyle. In Hapalops and especially 

 in Megalonyx this foramen is well in front of the occipital condyle. 



The paroccipital process, as in Hapalops, is inconspicuous, being 

 much less prominent than in Megalonyx. Between it and the stylo- 

 id Lindahl, J., Description of a skull of Megalonyx leidyi, n.sp., Trans. Amer. 

 Philos. Soc, vol. 17, n.s., p. 7, 1893. 



- r > Anthony, E., Eecherches anatomiques sur les bradvpes arboricoles, etc., 

 Ann. Sci. Nat., ser. 9 (Zool.), vol. 9, pp. 157-285, pis. 2-6, 1909. 

 27 Anthony, R., ibid., p. 178, 1909. 



