THE MyLaGAuLip&—RIGGs., 185 
premolar considerably enlarged, though the crown is little elon- 
gate antero-posteriorly. The third molar is retained, but is 
somewhat smaller than the second, and short roots are present 
throughout. Cope describes the mandibular dentition as fol- 
lows:* ‘* The crowns of the inferior molars, in the unworn con- 
dition, terminate in two crescents; that 1s, in elevated, anterior, 
external and posterior borders, with a transverse ridge equally 
dividing the space thus marked out and joining the notch in the 
external crest. This pattern resembles somewhat that of Aachi- 
thertum. Yhe first inferior molar (premolar) differs from the 
others in its superior size and its having the crescents more 
widely separated by a deeper external emargination. On attri- 
tion the spaces bounded by the enamel crests are enclosed 
by the junction of the extremities of these crests on the 
inner side of the crown. Further attrition results in three 
lakes within the crown and one notch of the external bor- 
der and two notches of the internal border. The anterior 
molar has two lakes in its posterior area and one large one 
in its anterior area. In old teeth there are successively one 
and no lakes left to interrupt the dentine. The inferior incisor 
has a wide shallow groove or concavity on Its external face.’’ 


(a) Lower dentition of Mesogau/us X 2. (6) The same of Protogaulus (Meniscomys, Cope) X 4. 
(c) Upper dentition of Hystrix refassa X 2. 
From this the affinities with Mesogaulus at once appear. In 
that genus the lateral emargination of the premolar-has almost 
disappeared, and the cleft is represented by the deeper middle 
fossette. The large anterior lake described in Proftogaulus is 
replaced by a smaller one in the later form; the deep postero-in- 
*Cope’s Tertiary Vertebra, p. 828. 
