MAMMALIA. 429 
teeth at the extremity of the upper jaw, none in the lower, and a dorsal fin ; 
thus, much nearer Hyperoodon and Heterodon than to Monodon. It is one 
of the rarest species of Cetacea, inhabiting the main ocean, seldom approach- 
ing the shores, and feeding chiefly upon pelagic molluscs. 
The genus Monodon, or narwhales, resembles the porpoises in the sphe- 
roidal head, but is deprived of a dorsal fin like Delphinapterus. The struc- 
ture of its skull brings it nearer the dolphins proper. The character by 
which it is distinguished from either one of them is the presence of tusks 
instead of teeth, originating from an alveolus common to both of the upper 
maxillaries and premaxillaries, which tusks are directed horizontally for- 
wards, and reach a length of eight or ten feet. ‘They are more or less 
twisted. Generally only one tusk is developed, the other remaining rudi- 
mentary. The mouth issmall; the dorsal fin rudimentary. The narwhales 
are chiefly found. in the polar seas, where they live in troops more or less 
numerous. 
Monodon monoceros (unicornis and bicornis) inhabits the seas of Green- 
land and .Spitzbergen. Another species is mentioned by Dewhurst, under 
the name of I. microcephalus, seen near Spitzbergen. 
_ Fossil remains of Monodon have been found in the upper tertiary beds 
of England and Russia, but have not yet been fully examined, so that the 
species are not determined and not compared with the living ones. 
ORDER 4. TLERBIVORA. 
Group 1. Sirenidia. 
_ The section of Sirenidia is composed of some aquatic mammals formerly 
placed among Cetacea, although by their herbivorous habits they have 
always constituted a natural group. Ancient writers have transmitted to 
us many fables respecting these animals; but modern investigation has 
removed the thick veil of ignorance which has enveloped this subject, 
especially with regard to the mermaids, those fabulous human-like beings, 
with the posterior parts of the body covered with scales, and terminated by 
afin. It is perhaps unnecessary to state that no such things as mermaids 
exist in nature. The seals, too, share with the Sirenidia the honor of giving 
rise to the mermaids and sirens of ancient and modern times. The pre- 
tended specimens usually exhibited in peripatetic or stationary museums, 
are generally fabricated from skins of monkeys artistically combined with 
those of fishes. 
The general form of the body of the animals comprised in this group is 
sub-cylindrical, tapering posteriorly, where it terminates by a horizontal fin 
similar to that of the whale. The head is somewhat detached from the 
trunk by a neck more or less apparent, and rises above the horizontal line 
of the body. The lower jaw is generally shorter than the upper one; both 
are furnished with teeth with a flat crown, more or less irregular, and 
adapted for the grinding purposes suited to their herbivorous habits. Instead 
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