Il8 PATAGONIAN EXPEDITIONS: PALEONTOLOGY. 



far as they can be made out, they differ in no important respect from those 

 of the other Santa Cruz family of the Litopterna, the Proterotheriidse. 

 The optic foramen is a large opening, situated low down upon the side of 

 the cranium and behind the orbit. Below and a little behind the optic 

 foramen, is the foramen lacerum anterius, also a large opening, especially 

 in the dorso-ventral diameter, but quite narrow and slit-like transversely. 

 There is no alisphenoid canal, and the foramina rotundum and ovale appear 

 to be confluent, a single large aperture perforating the alisphenoid inter- 

 nal to the glenoid cavity. A large, irregular opening in front of the tym- 

 panic doubtless includes both the eustachian canal and the foramen 

 lacerum medium. The foramen lacerum posterius is much smaller and 

 no carotid canal is discoverable. The postglenoid and stylo-mastoid 

 foramina are deep-set, but large and conspicuous. As already mentioned, 

 the condylar foramina perforate the basioccipital in a horizontal direction 

 and are concealed, in ventral view, by the overhanging condyles. 



In the adult animal, the two halves of the lower jaw are indistinguish- 

 ably fused together at the symphysis, a coalescence which takes place at 

 quite an early period. The horizontal ramus is very low and shallow 

 dorso-ventrally, but quite thick and stout transversely; the ascending 

 ramus is broad, but, aside from the coronoid, is not very high and the 

 condyle is raised but moderately above the level of the teeth. The angle 

 projects much posteriorly, and its free border is rounded and somewhat 

 incurved ; the masseteric fossa is large but very obscurely defined. The 

 condyle, which is almost sessile, is moderately convex ; on the hinder face 

 of the internal half is an articular surface for the postglenoid process of the 

 squamosal. The sigmoid notch is very narrow and the coronoid process 

 is very high, slender and somewhat recurved ; in the very young animal 

 the shape of the coronoid suggests that of the camel. The lineae obliquae 

 are very prominent and enclose quite a deep fossa, or depression, behind 

 the last molar. The inferior dental foramen is very large and there are 

 usually three mental foramina, beneath the canine and the first and second 

 premolars respectively. 



Of the hyoid apparatus (PI. XVII, figs. la, 2), two elements are pre- 

 served ; one is the relatively long, slender, cylindrical and outwardly 

 curved tympanohyal, which is not attached to the very small tympanic 

 bone, but to the posttympanic process of the squamosal. The other 

 element is the stylohyal (fig. 2), which has some resemblance to that of 



