ON THE STKUCTURE AND CLASSIFICATION 
lai.pe<l by the anterior cusp of the succeeding molar In old individuals (Mes. 
Mamm id. II, fig. 2), tlie central cusp is worn nearly to the level of the anterior 
misp, and the molars resemble tliose of Phascolestes. 
PHASCOLESTES, Plate IX, fig. 12. 
This genus is represented by the anterior portion of a left mandibular ramus 
(No. 47,741). Professor Owen proposed the name with some doubt as to its value, 
since he was inclined to associate the specimen with Peralestes, a genus repre- 
sented only by an incomplete maxillary fragment (p. 35, Mes. Mamm.) Such an asso- 
ciation seems to the writer unsafe and improbable on the follmving grounds, which 
will be made clear bv a comparison of figs. 8 and 12 : (1)» There are six molars, and 
possibly five premolars in the Peralestes maxilla, and eight molars and four premolars 
in the Phascolestes mandible; (2), The Peralestes (upper) molars have separate 
opposed i. e. internal and external cusps, the highest of which is internal, while in the 
Phascolestes (lower) molars there is no indication of the presence of separate opposed 
cusps; (3), We find in another genus, Peraspalax, a mandibular dentition of the very 
character which we .should anticipate to find opposed to the Peralestes dentition. 
There are some reasons for uniting Phascolestes with Amhlotherium. The worn 
molar crowns of the Phascolestes mandible indicate a rather older individual than that 
to which the A. soriciiium mandible belonged. The resemblances between the two 
genera, as seen upon the internal face, are in the molar, premolar and incisor patterns, 
and in the curvature of the alveolar border. The differences are still more important, 
viz. : that wliile the matrix impressions of the Amhlotherium molars show no trace of 
external cusps, the last three missing molars of Phascolestes have left impressions 
of .styloid external cusps similar to those of Sti/loilxm, or “the impressions of 
thret' successive long and slender cones,” as described by Professor Owen, (Mes. 
Mamm., p. 35). 
The principal chariicters of the Phascolestes mandible and dentition are the fol- 
lowing : The symphysis is long and the incisor alveolus is elevated. The incisors are 
cqtiidistant; is much the largest, the tip of the crown is wanting and the fang is faintly 
grooved ; is a small tooth with a slender fang and expanded crown ; 4 is slightly 
larger, and t, is nearly double the height of The canine is high, pointed and 
rcciwved, with a broad fang, grooved upon the inner surface. The first premolar is 
nu imcntary , has a double fang, a well-worn crown and a low posterior heel ; 
pm^ ii so 1 ange , with a high recurved crown and prominent posterior heel like that 
\ f f f ^ blunt conical crown, with a distinct cingulum 
hi els cu" M ^ ”<1 >«. have a 
hind the fir 7 “ anterior cusp and a posterior sloping heel. Be- 
stvloid cxtental on ‘Vtl casts of the 
Sty loid external cusps of three suceeeding molars. 
