1 8 PATAGONIAN EXPEDITIONS: PALEONTOLOGY. 



dorso-ventral diameter. There is no distinct foramen rotundum, the large 

 foramen ovale, which is entirely within the alisphenoid, probably having 

 transmitted both the second and third branches of the trigeminal nerve. 

 No alisphenoid canal is present. Loss of the tympanic renders the eusta- 

 chian foramen and foramen lacerum medium indistinguishable, but the 

 course of the eustachian canal is probably indicated by a long, well-defined 

 groove on the alisphenoid. The glenoid foramen is large and conspicuous. 

 The foramen lacerum posterius notches the anterior border of the exocci- 

 pital, and the condylar foramen is almost concealed by the condyle, when 

 the skull is viewed from below; it opens into a small, but deep and con- 

 spicuous fossa on the exoccipital. 



The mandible has considerable resemblance to that of Hyracodon, a 

 genus of rhinoceroses from the North American Oligocene ; the two rami 

 are coossified at the symphysis, which is narrow, of no great length and 

 deeply concave on the dorsal side. The horizontal ramus is elongate, 

 rather shallow dorso-ventrally, but quite thick and stout. The ascending 

 ramus is rather broad and quite high, the condyle being raised well above 

 the level of the teeth ; the angle, which is regularly rounded and thin, 

 projects prominently behind the condyle, with convex external and concave 

 internal face ; the masseteric fossa is quite small, but well defined, and is 

 situated very high upon the ascending ramus, reaching from the condyle 

 to the level of the alveolar border. The condyle is sessile, so that there 

 is no definite sigmoid notch, and is much extended transversely, especially 

 toward the inner side ; on the posterior face of the condyle, near the inner 

 end, is a considerable facet, which articulates with a corresponding surface 

 on the postglenoid process. The coronoid is high, rising much above the 

 condyle and, when the jaws are closed, projecting above the zygomatic 

 arch; it is thin, but moderately broad and is either erect, or inclined 

 slightly forward. The inferior dental foramen is situated behind and a 

 little below ms, and there is a large mental foramen on each side below 

 the canine or px. 



Vertebral Column and Ribs (PI. I). — In no individual of the Protero- 

 theriidae has a complete series of vertebrae been found, as yet. There is, 

 however, reason to believe that the formula was as follows: C. 7, Th. 15, 

 L. 6-7, S. 6, Cd. ?. In one specimen belonging to the American Museum 

 (No. 9290), the neck is preserved with all the vertebrae in place and 15 

 trunk vertebrae are also present. The neck is decidedly short and quite 



