THE ELEPHANTS AND SEA COWS 223 



addition of a hypocone and the lower molars present the usual 

 tuberculo-sectorial structure with loss of the paraconid and 

 elongation of the talonid. In each series the third molar has a 

 pronounced posterior basal cingulum and it is from the con- 

 stant growth and extension of this that the elongation of the 

 tooth so pronounced in later races is brought about. 



The Elephant molars are then no exception to the general 

 method whereby adaptation of the tooth to an extreme brows- 

 ing or grazing character is brought about. Indeed the molar 

 does not strikingly differ from that of the Rodent Capybara 

 and the same method of evolution of molars has already been 

 studied in less pronounced form in the Pig, Bear, Baboon and 

 Kangaroo. 



SIRENIA 



Closely related to the Proboscidea are the marine browsing 

 Mammals, the Manatee of Florida and the Dugong of the Indian 

 Ocean. These strange creatures also known as Sea-cows feed 

 upon seaweed, eel-grass and aquatic plants. 



The skull of the Dugong like that of the Elephant is markedly 

 specialized and although the molars when first erupted show an 

 occlusal surface of masked selenodont appearance this surface 

 soon wears away and the teeth remain as simple hypsodont col- 

 umns with permanently growing roots. In the upper jaw the 

 incisors, apart from certain vestigial rudiments buried under 

 a horny casing, are reduced to two long tusk-like teeth which in 

 the female do not project beyond the gum (see Fig. 80) but in the 

 male resemble Rodent incisors in that they possess persistently 

 growing roots and have no enamel upon their posterior surfaces. 

 In the mandible there are four or five rudimentary antemolar 

 teeth on each side imbedded in the sloping surface of the symphy- 

 sis which are functionless and are covered by a horny plate 

 opposing the obliquely placed premaxillary part of the palate. 

 Of the cheek teeth there are altogether five or six on each side 

 with persistently growing roots all devoid of enamel and all 

 cylindrical except the last which is elongated and grooved on 



