430 ZOOLOGY. 
of the b owing-holes of Cetacea, we find here nostrils constructed like those 
of other mammals. There exists no external ear; the eyes, proportionally 
small, are provided with a nictitating membrane, which is wanting in Ceta- 
cea; the upper lip and snout are beset with thick beard-like bristles; the 
skin itself is generally smooth, and deprived of hair. Of the locomotive 
organs, the anterior ones alone are present, as in Cetacea, and similarly 
constructed for swimming; for this its adaptation is very great. The pos- 
terior limbs are completely wanting. Sirenidia live in society, not far from 
the shores, and at the mouths of rivers. They often go ashore to feed upon 
marine or aquatic plants, and may occasionally drag themselves on dry land, 
which Cetacea never do. ) 
Several fossil genera of this group have already been described, and we 
may expect some more to be discovered hereafter. The living genera are 
only three, Manatus, Halicore, and Rytina, comprising a very small number 
of species. We shall first introduce the extinct members, as the oldest data 
of the history which we endeavor here to relate. 
Fam. 1. DiInotHERIDz. The genus Dinotherium is founded upon a gigan- 
tic fossil from the tertiary beds of Germany, which created a great sensation 
at the time of its discovery. Different opinions have been entertained with 
regard to its true zoological affinities. First placed near the tapir and mas- 
todon, it is now generally associated to the Manati and other Sirenidia. At 
any rate, the Dinotheria are pachyderms, and were never mistaken as such. 
And when brought in the same group with Manati, this fact ought to have 
revealed to us the affinities of the so-called herbivorous Cetacea with pachy- 
derms, of which they form the lower grade, and among the latter the Dino- 
theria are the lowest. Their lower jaw is terminated by two tusks, curved 
downwards and backwards. The existence of great sub-orbital holes, and the 
form of the nasal bones, have induced the belief that Dinotherium was pro- 
vided with a proboscis similar to that of the tapir and elephant. The molar 
teeth, five above and five below, remind us of those of the tapir and manatee. 
Several species have been described from the tertiary beds of Hurope. 
D. giganteum is the largest and best known. Its habits are thought to have 
been similar to those of the Manati; it frequented the mouths of rivers, 
feeding upon aquatic plants; the tusks were occasionally used to force 
them out of the ground. A species of Dinotherium has been announced 
from the eocene of South Carolina. 
The genus Metaxytherium possesses all the osteological characters of the 
Dugong, as also its tusks, but the grinding teeth resemble those of the 
Manati. Although several species are known to have existed in Hurope 
during the tertiary epoch, they are imperfectly characterized. Some of 
their remains have previously been referred to the genera Hippopotamus and 
Manatus, until Christol created the genus which we here record. 
The genera Halitheriwm and Pygmeodon come near the Manati and 
Dugong, but are too imperfectly known to allow us to give a more detailed 
account of their history. 
The genus Cheirothertum, with a skull and skeleton constructed like the 
skull and skeleton of the Dugong and Manati, is provided with teeth, 
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