1913] 



Stock: Nothrotherium and Megalonyx 



345 



They are narrowed by the constriction of the cranium above 

 the base of the zygoma and indicated by the arch of the coronal 

 suture. Their surfaces show but a faint indication of the tem- 

 poral ridges, which swing back from the widest part of the 

 frontals to the median line, where they approach each other 

 at the coronal suture, but do not meet. In Megalonyx jeffersonii 

 these ridges are more pronounced and meet in the median line 

 at the coronal suture. This difference may be due in part to 

 sex or to age. There is no blunt postorbital process as in 

 N. escrivanense. A shallow depression occurs posterior to the 

 end of the temporal ridge. On the orbital portion of the 

 frontal a sharp undulating ridge, which continues posteriorly 

 over the temporal, curving outward to form the inner border of 

 its zygomatic process, overhangs the orbitosphenoidal region, as 

 in M. jeffersonii. 



On the parietals the temporal ridges swing outward to the 

 widest portion of these bones above the posterior end of the 

 superior border of the zygoma. The parietals are distinctly 

 swollen at the posterior end of the temporal ridges. Between 

 the temporal ridges the parietals are flattened and show only a 

 suggestion of a sagittal crest, which is restricted to the posterior 

 half of the parietal region. In Megalonyx jeffersonii the rugged 

 sagittal crest extends from the coronal suture posteriorly to the 

 lambdoidal crest, which in this form is also more sharply 

 defined than in Nothrotherium graciliceps. This difference is 

 probably not due to age. On the vault of the parietals below the 

 temporal ridges are several foramina, comparable to the "venous 

 foramina" of Leidy's material of M. jeffersonii. 



The temporal is very long, with a comparatively small vertical 

 width. It is depressed above the mastoid area and close to the 

 lambdoidal suture. The trihedral zygomatic process is short, 

 with the superior border distinctly convex at the middle and a 

 slight corresponding concavity on the inferior border. In the 

 Brazilian species these two borders are straight. The zygomatic 

 arch of Megalonyx differs greatly from that of Nothrotherium 

 in bending more markedly downward and outward from the 

 cranial wall. A vascular foramen pierces the middle of the 

 lateral face of the zygomatic process. 



