MAMMALIA. 429 



teeth at the extremity of the upper jaw, none in the lower, and a dorsal fin ; 

 thus, much nearer Ilyperoodon and Iletcrodon 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 narvvhales, 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 is small ; the dorsal fin rudimentary. The narwhales 

 are chiefly found in the polar seas, where they live in troops more or less 

 numerous. 



Monodon monoceros {iinicornis and hicornis) inhabits the seas of Green- 

 land and Spitzbergen. Another species is mentioned by Dewhurst, under 

 the name oi M. microcephalus, seen near Spitzbergen. 



Fossil remains of Monodon have been found in the upper tertiary beds 

 of England and Kussia, but have not yet been fully examined, so that the 

 species are not determined and not compared with the living ones. 



Order 4. IIerbivora. 

 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 

 a fin. 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 generali}^ shorter than the upper ojie ; 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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