210 
TERTTAllY VERTEBRATA OF THE FAYtEVI. 
opening, and beneath the alveolus-like depression noticed above, there is a rounded 
mass, whicli seems to be homologous with the bony capsule in which the successional 
teeth develo]) in the Manatee. Probably, therefore, in the mandible at least of the 
])resent species, the beginning of a molar succession similar to that seen in the Manatee 
had already been established. 
Lower Dentition. — In no specimen are the lower teeth well preserved, and in the 
most nearly complete example there are only three greatly worn and broken molars 
in situ. Phe following account of the lower teeth is therefore deduced merely from 
the alveoli in the jaw figured on PI. XX. figs. 2, 2 a, and, when more satisfactory 
specimens are found, may have to be revised. In the downwardly turned alveolar 
surface of the symphysial region there are four pairs of alveoli, probably indicating 
the existence of three pairs of incisors and a canine. The most anterior pair of 
sockets are shallow and not very well defined, so that not improbably the anterior 
tooth was deciduous, and its place covered with a horny sheath as in Manatus and 
probably IlaJitherium. On the other hand, the remaining three pairs of alveoli are 
so deep and sharply defined that most likely their teeth were functional. Behind 
these symphysial alveoli come thirteen others, increasing in size from before backwards. 
The posterior six sockets undoubtedly bore three double-rooted molars (as can be 
seen in another specimen), and the remaining seven probably belonged to three 
anterior single-rooted teeth (1 premolars) and two posterior double-rooted teeth. If 
this interpretation be correct, the dental formula of this mandible is i. 3, c. I, inn. 3, 
m. 5, regarding the double-rooted teeth as molars. As mentioned above, it seems 
not impossible that the number of molars would subsequently be added to from 
behind. 
The only lower molars available for examination are greatly worn, but seem to 
have consisted of a pair of transverse crests and a small posterior talon. 
Mandible of Eotherium. — A mandible (text-fig. 67) from the white limestones of 
the Mokattam Hills probably belongs to the same species and perhaps to the same 
individual as the skull above referred to Eotherium eegyptiacum. It differs very little 
in form from the mandible of Eosiren, having the same strongly deflected anterior 
region ; the symphysis {sym.) is greatly thickened, though much less so than in 
Eosiren. The chief difterence between the two is that the alveoli of the anterior 
teeth are better marked. Unfortunately, only the three molars arc present, and 
the interpretation of the dentition from the alveoli is o[)en to much uncertainty. 
The three pairs of incisors were large teeth, the anterior ones [i. I) being in contact 
with one another in the middle line, and directed almost forwards. On the upper 
surface of the symphysis between the alveoli of the second and third pairs of incisors 
(^. 2, ^. 3) and of the canines (c.) is a rough surface which was probably covered 
with the horny substance which eventually displaced and replaced the teeth. A little 
