414 PAT AGONIAN EXPEDITIONS : PALAEONTOLOGY. 



abrasion that dp4 long remained in function and was not replaced until 

 after m3 had come into use. Ameghino states that there are two milk- 

 premolars, one of which has no successor (94", 67). 



The skull (Plate LXVI, figs. 1-6) concerning which some significant 

 points remain to be determined, is, in general appearance, much more like 

 that of Erethizon than that of Coendou, which is principally due to the ab- 

 sence of the inflation of the frontal and nasal regions so characteristic of 

 the latter. A short, but well defined sagittal crest, which is much more 

 prominent than in Erethizon, occupies the posterior half of the parietal 

 zone, passing forward abruptly into a lyrate sagittal area, which is much 

 narrower and has more nearly parallel sides than in the last-named genus. 

 The nasals are longer and more convex transversely thari in the latter, 

 and the dorsal branch of the maxillary zygomatic process is broader, or less 

 antero-posteriorly compressed than in either of the recent genera. The 

 rostrum has about the same relative length as in Erethizon, but is of more 

 uniform width, not narrowing so much ventrally and thus, when viewed 

 from below, is more as in Chcetomys than in either of the other modern 

 genera. The incisive foramina are large, resembling those of Erethizon. 

 None of the specimens shows the shape or limits of the posterior nares. The 

 pterygoids have a wider ventral surface than in the recent genera and do 

 not extend to the tympanic bullse ; apparently the pterygoid fossa is imper- 

 forate, though the skulls are all so damaged here as to leave this somewhat 

 uncertain. The tympanic bullae are far smaller than in Erethizon or Coen- 

 dou, with meatus formed by short, irregular tube, much as in the former. 



The mandible differs in several respects from that of the recent genera ; 

 the symphysis is shorter and the chin more vertical, and the diastema is 

 relatively much shorter ; the ascending ramus is not set quite so far for- 

 ward and the coronoid is much less reduced, rising above the level of the 

 condyle, while the sigmoid notch is wider and shallower; the masseteric 

 crest is much more prominent and the fossa much deeper and better 

 defined ; the pterygoid crest, on the other hand, is less developed, so that 

 the ventral border of the angle is narrower and less inflected ; the angular 

 process is shorter and the notch between condyle and angle shallower. 

 The alveolus of the incisor forms a broad, shelf-like projection from the 

 inner side of the ascending ramus, rising much higher than in the recent 

 genera and bringing the inferior dental foramen to a much more elevated 

 position, a little below the condyle. 



