MANATIDE 217 
backwards considerably behind the orbits. Their sides are formed 
by the ascending processes of the premaxille below, and by the 
supraorbital processes of the frontals above, no traces of nasals 
being found in most skulls, though these bones are occasionally 
present in a most rudimentary condition, attached to the edges 
of the frontals, far away from the middle line, in a position 
quite unique among the Mammalia. In front of the narial aper- 
ture the face is prolonged into a narrow rostrum, formed by 
the premaxille, supported below and at the sides by the maxille. 
The under surface of this is very rugose, and in life covered by a 
horny plate. The rami of the mandible are firmly united together 
at the symphysis, which is compressed laterally, slightly deflected, 
and has a rugose upper surface; to this another horny plate is 
attached, which, with that of the upper jaw, functionally supplies the 
Fic, 72.—Skull of African Manatee (Manatus senegalensis). 4 natural size. 
From Mus. Roy. Coll. Surgeons. 
place of teeth in the anterior part of the mouth. In the young 
state there are rudimentary teeth concealed beneath these horny 
plates, which never penetrate through them, and must therefore be 
quite functionless, and altogether disappear before the animal is full- 
grown. There is besides on each side of the hinder part of both 
upper and lower jaws, a parallel row of molar teeth, similar in 
characters from the beginning to the end of the series, with square 
enamelled crowns raised into tuberculated transverse ridges, some- 
thing like those of the Tapir and Kangaroo. The upper teeth have 
two ridges and three roots; the lower teeth have an additional 
posterior small ridge or talon, and but two roots. These teeth 
succeed each other from before backwards, as in the Proboscidea, 
those at the front of the mouth being worn out and shed before 
those at the back are fully developed. There are altogether about 
eleven on either side of each jaw, but rarely more than six are 
