224 STRENIA 
genus, which may be identical with TYrachytheriwm of the French 
Miocene, better preserved remains have subsequently been described 
by Delfortrie. These show that the rostrum is more elongated 
than in Hulitheriwm, but the skull is otherwise very similar, as are 
the molar teeth. The incisors are very large, exserted, strongly 
compressed, almost sabre-like, rounded on the upper or anterior 
surface, sharp below, concave on the external and convex on the 
inner side, and transversely striated. 
Pachyacanthus from the Miocene of the Vienna basin is also, ac- 
cording to Van Beneden, another form of Sirenian, of which, however, 
the skull is not known. In various Miocene marine formations of 
the United States of America other remains of Sirenians have 
been found, but mostly in such a fragmentary condition that they 
afford at present little evidence of the early history of the group 
in that country. A more satisfactory discovery is that of a 
nearly complete skull and some bones from a Tertiary limestone 
formation in Jamaica. It is of smaller size than the Manatee, 
and, so far as the teeth are concerned, of a still more generalised 
character than Halitherium, the dentition being apparently i 3, ¢ 4, 
ptm ( rs) =48. The incisors are small, not developed into tusks ; 
the canines (wanting in all existing Sirenians) are rather larger 
than the incisors, judging by the sockets; and the molars are 
bilophodont, and covered with enamel. It has been described 
by Sir R. Owen under the name of Prorastomus sirenoides. Some 
writers regard this genus as the type of a distinct family—the 
Prorastomatide. Unfortunately we have no knowledge of the geo- 
logical antiquity of the formation in which it was embedded. Lastly 
must be mentioned the Hotherium egyptiacum, Owen, founded on the 
cast of a brain, with a small quantity of surrounding bone, discovered 
in the nummulitic limestone of Eocene age in the Mokattam Hills, 
near Cairo. The brain is narrower than in Muanatus, and resembles 
that of Haktherium. This is of interest as the most ancient known 
evidence of any Sirenian whose age has been geologically deter- 
mined. Teeth from the same deposits referred to JJanatus not 
improbably belong really to Eotherium. 
The few facts as yet collected relating to the former history of 
the Sirenia leave us as much in the dark as to the origin and 
affinities of this peculiar group of animals as we were when we only 
knew the living members. They lend no countenance to their 
association with the Cetacea, and on the other hand their supposed 
affinity with the Ungulata, so much favoured by modern zoologists, 
receives no very material support from them. 
Bibliography of Sirenia.—J. F. Brandt, Symbol Sirenologicee, St. Petersburg, 
3 fasciculi, 1846-61-68—an exhaustive account of the anatomy, aftinities, and 
literature of the group, with copious illustrations of the osteology of Rhytina. 
