244 THE MAMMALIA. 



the word ; the latter are a very ancient branch of 

 the Hoofed Animals. 



Of the living Sirenia the Manatus shows the 

 fullest dentition with a change of teeth. It points 

 to an old Tertiary group found in Jamaica, the 

 Prorastomus sirenoides, whose molars are genuine 

 ridged teeth. 



The other and more perfect line ends in the 

 present period with the so-called Steller's Sea-Cow 

 (Rhytina stelleri), which has recently become ex- 

 tinct. It had no true teeth for masticating pur- 

 poses, but, in place of molars, had large fibrous 

 structures on the gums, one on each side of each 

 jaw. These structures occur also in two of the 

 living species, but are less large. The dugong 

 already shows a considerable loss of teeth, but by 

 possessing them stands nearer to the earlier form 

 of Sirenia, which leads in a direct line back to the 

 Eocene Halitherium. The dental formula is : 



_ 



When comparing the genuine Hoofed Animals 

 with their ancestors, it was seen that the loss of 

 one or two toes took place as early as in the first 

 Tertiary division. It was only single genera, such 



