PLANT-EATING MAMMALS 173 



SEA-COWS (SlRENlA) 



Sea- Cows are herbivorous mammals which have taken to an 

 aquatic life, feeding along the shore and, in one species, travel- 

 ling up rivers for a considerable distance. Only two forms now 

 exist the Manatee and Dugong. 



The Manatee (Manatus] (see vol. i, p. 101) is found on both 

 coasts of the tropical parts of the Atlantic, and lives for the 

 most part in the larger rivers, which it may ascend for hundreds 

 of miles. Only fore-limbs are present, and though these help to 

 some extent in seizing food, the chief prehensile organ is the 

 upper lip, which is well developed and of remarkable shape. 

 Deeply excavated in the middle, it possesses a sort of pad on 

 each side, and the two pads can be brought together for the 

 purpose of grasping food, which is held fast by their bristly 

 surfaces, and then drawn back into the mouth. Canine teeth 

 are entirely absent, and the adult possesses no incisors, though 

 in the young animal vestiges of these are to be found under the 

 thick horny plates with which the front of the jaws is armed. 

 The roof of the mouth is furnished with similar plates. The 

 cheek-teeth are in the form of grinders, with broad, square crowns 

 marked by transverse ridges. Of these teeth 44 in all are devel- 

 oped, but not more than 24 of these are as a rule in use at once, 

 for by the time the hinder ones are in place those in front of them 

 have dropped out. 



The Dugong (Halicore] (fig. 421) is essentially a marine 

 animal, ranging round the shores of the Indian Ocean to the 

 coast of Australia. The upper lip is not specialized for prehen- 

 sion as in the Manatee, but the lower lip is relatively larger 

 and much swollen. One marked peculiarity is found in the front 

 parts of the jaws, these being bent abruptly downwards. As 

 before, they are armed with crushing horny plates. Incisors are 

 better developed than in the Manatee, for two are present in 

 the male in the form of long sharp tusks. These are developed 

 within the jaw of the female, but never cut the gum. The re- 

 maining teeth are simple rounded molars with flat, smooth, rounded 

 crowns. Not more than twenty are present at any time, and they 

 appear to be degenerating structures, as shown by the absence 

 of enamel. It is interesting in this connection to observe that 

 a large toothless Sirenian, Rhytina, inhabited the coasts of the 



