348 University of California Publications in Geology [Vol. 7 



the right bulla is pierced by an elongate opening, which appears 

 not to have been present in N. escrivanense. 



A small projection of the alisphenoid is suturally defined on 

 the left side, where it lies below the overhanging frontal ridge. 

 On the left side, where the tympanic bulla has been removed, 

 the alisphenoid is seen to form the roof of the auditory capsule. 

 The orbital fissure is large. The large combined opening of the 

 foramen rotundum and foramen ovale is about as far posterior 

 to the orbital fissure as the optic foramen is anterior to it. 

 Since the outer wall of the bulla, formed by the pterygoid, pro- 

 jects ventrally from the margin of the alisphenoid, the two 

 foramina open externally through the pterygoid. In Megalonyx 

 these two foramina open separately. 



The concave basisphenoid narrows anteriorly, the apex being 

 indicated by a foramen opening above the vomer. Leidy states 6 

 that in Megalonyx jeffersonii the sphenoidal surface is plane, and 

 cites a similar foramen above the vomer. The median portion 

 of the basioccipital is relatively much more prominent than in 

 N. escrivanense. The condylar foramen is small. 



The glenoid fossa is but slightly concave. The base of the 

 zygoma has a much greater anteroposterior diameter than in 

 M. jeffersonii. Below the rugose mastoid area is the large 

 stylohyal process. Anterior to the base of this process is the 

 large jugular foramen. In Reinhardt's figure 7 of the skull of 

 Nothrotherium escrivanense a much smaller foramen is indicated 

 in this region. The anteroexternal face of the stylohyal process 

 is grooved inferiorly. Superiorly this groove is converted into a 

 closed canal, the stylohyal canal, which leads to the jugular 

 foramen. Extending anterior to the process is a vertical plate 

 which abuts upon the posterior wall of the bulla. The stylohyal 

 process is continuous dorsally and posteriorly with the trans- 

 verse crest of the occiput (fig. 4), which is midway between the 

 lambdoidal suture and the foramen magnum. In Megalonyx 

 jeffersonii this crest is one-third the vertical distance from the 

 lambdoidal suture. 



6 Leidy, J., A Memoir on the Extinct Sloth Tribe of North America, . 

 Smithson. Contrib. Know]., vol. 7, p. 10, 1855. 



7 Reinhardt, J., op. cit., Tab. 1, fig. 2. 



