ORDER SIRENIA. I3II 



allied to Halitherium, but with a fuller dentition, and without the 

 marked deflection of the extremities of the jaws. The dentition 



is I. -, C. -, Pm.-\-M. ; the upper incisors are not tusk-like ; 



31 7 or 8 r 



and the cheek-teeth have simple transverse ridges somewhat like 



those of the Ungulate genera Dinotherium and Listriodon. Eo- 



therium is definitely known by a cast of the brain-cavity from the 



Eocene of Egypt, but teeth from the same deposits described under 



the name of Manatus may perhaps belong to it ; its affinities cannot 



yet be properly determined. Probably allied to Halitherium is 



Crassitherium from the Pliocene of Belgium ; while Dioplotherium 



from the Miocene of South Carolina is regarded as intermediate 



between Halicore and Halithei'ium. 



Family Rhytinid^e. — The Rhytinidce are known only by the 

 huge Rhytina gigas, or Steller's Sea-Cow, which was formerly very 

 abundant on Behring and Copper Islands in the North Pacific, 

 but is commonly supposed to have been exterminated about 1768, 

 although a few individuals may have lingered on to a considerably 

 later date. The Rhytina attained a length of from 20 to 25 feet, 

 and is characterised by the entire absence of teeth, their function 

 being supplied by horny plates (cornules) on the palate. The head 

 is very small in proportion to the length of the body ; and has the 

 premaxillse and mandibular symphysis moderately deflected. The 

 skin was naked, and covered with a rugged epidermis resembling 

 the bark of a tree. Nearly entire skeletons have been obtained 

 from the peat of Behring Island. 



Family Manatid^e. — In Manatus, the only known representative 

 of this family, the premaxillae and mandibular symphysis (fig. 1 1 82, b) 

 are not deflected, and there are no functional incisors. The cervical 

 vertebras are reduced to six ; the cheek-teeth, which are coated with 

 enamel and carry two transverse ridges, may be as many as eleven 

 in each jaw, although it is seldom that more than six are present 

 at any one time. The Manatees are inhabitants of the mouths 

 and estuaries of the great rivers discharging into the two sides of 

 the Atlantic. No fossil forms are known which can be referred 

 with certainty to this family, although the type generic name has 

 been applied to certain teeth from the Eocene of Egypt already 

 mentioned. 



As Sirenians of uncertain affinity may be mentioned Chronozoum 

 from the later Tertiary of New South Wales ; Hemicaulodon from 

 the Eocene of Shark River ; Pachyacanthus from the Miocene of 

 Vienna ; and Trachytherium (with which Rhytiodus may perhaps be 

 identical) from that of France. 



