436 PATAGONIAN EXPEDITIONS: PAL/EONTOLOGY. 



a contact with the lachrymal. The dorsal branch of the maxillary zygo- 

 matic process is very broad, making the outer wall of the infraorbital fora- 

 men far wider than in the recent genus. 



The nasals have much the same foliate shape as in Viscaccia, but in 

 their anterior half these bones are much less convex and have no such 

 inflated appearance; while the free ends are bluntly pointed and not 

 notched, but are separated for a considerable distance from the premaxil- 

 laries. The latter are very similar to those of the recent genus, but the 

 masseteric fossa is shallower and of a different shape ; the ascending rami 

 are long, extending over the maxillaries to a suture with the frontals, the 

 shape of which varies in the different species. The hard palate is of tri- 

 angular shape, narrowing forward ; much of it is formed by the palatines, 

 which have a smooth, dense surface, in marked contrast to the almost 

 lace-like texture of the maxillary palatine processes ; the maxillary portion 

 of the palate is less concave than in Viscaccia and the median ridge is 

 lower, while the ventral surface of the palatine bones is broader. The 

 posterior nares are considerably smaller than in the modern genus and there 

 is no such median spine from the front border. The pterygoid fossae are 

 large and are so perforated as to be in communication with the orbits. 



The mandible is very much like that of Vjscaccia, with certain signifi- 

 cant differences ; the condyle is narrower and more elongate antero-pos- 

 teriorly; the coronoid, though very small, is much less reduced, rising 

 nearly as high as the condyle, from which it is separated by a far narrower 

 and shallower sigmoid notch. The masseteric fossa is very deep and has 

 quite the same elevated position on the ascending ramus as in the modern 

 genus, but is considerably larger; the masseteric crest, which in the ex- 

 isting genera of the family is almost obsolete, in Perimys is very prom- 

 inent, especially in the dentary region of the jaw, becoming less distinct 

 posteriorly ; the pterygoid crest also is better developed, making the angle 

 more concave on the inner side and giving it a much wider ventral margin. 

 The symphysial region of the jaw is less procumbent and the chin rises 

 more steeply from the ventral border of the horizontal ramus than in the 

 existing genera. 



Unfortunately, little is known of the skeleton, other than the skull, but 

 enough remains to show that there are certain not unimportant differences 

 from all of the recent genera of the family. The parts to be described 

 are all from the quite small species, P. scalaris and P. puellus. The 



