1 6 PATAGONIAN EXPEDITIONS! PALAEONTOLOGY. 



so visible. This is one of the few characters in which Stegotheriuui is 

 more like Dasypns. The condyles are widely separated and very oblique, 

 rising toward the outer sides; they are less completely sessile than in 

 Tain, being demarcated from the occipital plane by deeper fossae, and on 

 the ventral side are much more distinctly separated from the body of the 

 basioccipital. In shape, they are transverse semicylinders ; the condylar 

 elevation extends out laterally to the borders of the occiput, where it is 

 separated from the squamosal only by a very narrow strip of the periotic, 

 but the articular surface does not extend so far, and is quite deeply 

 invaded by a lateral sulcus ; the ventral portion of the surface is produced 

 farther laterally than the dorsal part. There is no paroccipital process nor 

 any space for such a process between the condylar ridge and the periotic. 

 Though the sutures between the supraoccipital and the exoccipitals have 

 disappeared, it may reasonably be assumed that, as in Tatu, the former 

 was very large, making up nearly the whole of the occipital surface, and 

 forming the dorsal margin of the foramen magnum and the whole of the 

 lambdoidal crest ; a small portion of the bone is bent over upon the roof 

 of the cranium, though less conspicuously so than in Tatu, owing to the 

 depth of the median notch. 



The periotic is exposed as a narrow strip between the exoccipital and 

 the squamosal ; it has about the same relative breadth as in Tatu and is 

 perforated by a similar vascular foramen as in the latter, but has no such 

 groove leading into it. Just above the outer edge of the condylar eleva- 

 tion is a small but deep depression in the periotic, which is not present in 

 the modern genus. The mastoid process is short, but distinct, and is 

 closely applied to the posttympanic process of the squamosal ; distally it 

 ends in a concavity for the hyoid. The auditory, or petrosal, portion of 

 the periotic is quite large and very superficial in position and is not en- 

 tirely concealed from view by the tympanic. The latter is small and 

 forms an incomplete ring of bone, C-shaped when viewed from the ven- 

 tral side ; it is almost exactly like that of Tatu, but is a little broader, 

 covering more of the periotic. 



The basisphenoid, though short and broad, is somewhat longer than 

 the basioccipital and narrows forward slightly ; it has a raised line near 

 each lateral border, and these lines converge anteriorly, enclosing a shield- 

 shaped area, with a shallow fossa on each side of the median line, thus 

 giving to the whole bone a very different appearance from the simply 



