202 
ON THE STKUCTURE AND CLASSIFICATION 
mo»o»lc mammals, the nearest relationship suggested is to AmNoa^um. Uptocla. 
,lu. approaches this genus in its dental formula (p ».4 »6), and d, tiers from it and .ts 
allies in the suppression of the anterior lobe of the molars which is replaced by a cm- 
gulum cu.sp. 
The ramus has a nearly straight lower border ; below the first premolar it curves 
upwards rapidly. The outer surface is convex and shows two foramina beneath 
p;»2 and m,. The alveolar border is depressed below the premolars and rises in the 
middle of the molar series, this curvature resembling that in Phascolestes. Professor 
Owen discovered traces of two incisor sockets; these teeth were probably small, 
compactly placed, and not widely separated from the canine. Judging by the socket, 
the canine was a large, erect tooth. The succeeding four teeth are evidently pre- 
molars ; they increase rapidly from the first to the fourth ; there is a minute cusp 
ui)on the anterior slope and a posterior heel which varies in width ; the absence of 
an external cingulum separates them from the molars. The premolars of mesozoic 
mammals invariably lack the external cingulum. There are probably six bifanged 
molars, two of which filled the space between nii and the coronoid process. The 
first molar has a conical, central cusp and a broad posterior heel ; on the anterior 
8loi)e, the cingulum forms a basal cusp, and, disappearing below the main cusp, 
reappears at the side of the posterior heel. In mg, m 3 and the main cusp is tall, 
slender and slightly recurved, while the posterior heel is very broad and much worn. 
PERAMUS. Plate VIII, fig. 6. 
In Peramns and the genera which, although not closely affiliated, will be con- 
sidered along with it, viz, Spalacotfierium, Peralestes and Peraspalax, all from the 
I’urbcck, we meet a new principle in the construction of the molars. There are 
still three cusps, but these cusps are not arranged in the same fore-and-aft line ; 
owing eitlicr to the rotation inwards of one or more of the cusps, or to the elevation of 
the internal cingulum. We find the crown of the tooth broadening to support 
internal and external or opposed cusps. 
1 lie chief materials for the determination of the characters of this genus consist 
of portions of two mandibular rami. In the type specimen (No. 47,742, Brit. Mus.), the 
])osterior half of the ramus is preserved; in the second specimen (No. 47 , 743 ) the anterior 
half of the ramus is prc.served; both specimens preserve the first molar intact with 
some of the adjoining teeth, and we are thus bnabled to place these halves together 
and reconstruct the jaw, as seen in figure 6. Another mandibular fragment (No. 47,743), 
preserving the canine and portions of the premolars and incisors, has been placed with 
Peramus by Professor Owen (PI. II, fig. H), but in the absence of a molar tooth, 
this determination is uncertain on the following grounds : the fifth and sixth pre- 
molars of this specimen have conical crowns, with a narrow base, whereas in the type 
specimen, pm 3, is bifanged with a very broad base and anterior and posterior basal 
