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 united 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 
(PL LXIII, fig. 7), wanting in others (PL LXIV, fig. 1). 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 (PL LXIII, figs. 4 a, 5^), 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. PL LXIII, figs. 4 and 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 (PL LXIII, fig. 4 b), 
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. PL LXIII, figs. 
