426 PATAGONIAN EXPEDITIONS: PALAEONTOLOGY. 



minute accessory cuspule is developed in some species. The crown ter- 

 minates in a thick blunt point. The anterior root is very oblique and 

 narrower transversely than the posterior root. The principal wear is on 

 the postero-internal face of the crown, where the tooth shears against the 

 anterior blade of the lower sectorial. The molars decrease rapidly in size 

 posteriorly. Each is triple-rooted, with two roots on the buccal and one 

 on the lingual side. The first is fully quadritubercular ; the second has 

 an incipient hypocone ; the third and fourth are tritubercular. The 

 crowns are bunodont. The protocone and hypocone are unite.d respec- 

 tively with the paracone and metacone by transverse ridges (cf. PI. 

 LXIII, fig. 7), and the latter cusps with each other by a sharp trenchant 

 ridge, which passes over the external cusps, as in Petaurus (PI. LXV, 

 fig. 4). An external cingulum is faintly indicated in some specimens 

 (PI. LXIII, fig. 7), wanting in others (PI. LXIV, fig. i). A rather broad 

 anterior cingulum is developed on the first molar. 



The median lower incisors are procumbent and lanceolate, with the 

 outer edge of the enamel attenuated (PI. LXIII, figs. 4^, 50), and occasion- 

 ally notched by accidental fractures received during life. The enamel 

 layer is restricted to the anterior face of the crown. Internally, it is re- 

 enforced by a thick rib of dentine. The enamel does not grow persist- 

 ently but covers a limited area, which decreases in size as the tooth wears 

 down (cf. PI. LXIII, figs. 4 an$ 5). In shape the incisors resemble those 

 of the Macropodidae. 



Following the incisor are four single-rooted vestigial teeth. The an- 

 terior two are remarkably procumbent in P. minutus (PI. LXIII, fig. 46), 

 possibly less so in P. lepidus. The third is not retained in any of the 

 specimens in the collection. The crown of the fourth is antero-posteri- 

 orly elongated and overhangs in front. The posterior premolar is a large 

 double-rooted tooth elevated to about the same extent as the molars. 

 The crown supports a prominent central cusp and more or less elevated 

 anterior and posterior accessory cuspules. These have been used by 

 Ameghino in defining the genera Metaepanorthus and Paraepanorthus. 

 The former he characterizes by the presence of well-defined anterior and 

 posterior accessory cuspules on the posterior premolar, and the latter by 

 the occurrence of the anterior cuspule only. The presence or absence of 

 these structures can hardly be a matter of generic importance, as they vary 

 in size and prominence within the limits of a species (cf. PI. LXIII, figs. 



