VERTEBRATA: MAMMALIA. 683 



permanent incisors, which are entirely concealed in the jaw in 

 the females, but which increase in size in the males with the 

 age of the animal, till they become pointed tusks. Both upper 

 and lower jaws are strongly bent down in front, and the de- 

 flexed portions of the jaws bear horny plates. The anterior 

 extremities are nail-less, and the tail-fin is crescentic in shape. 

 In their general appearance and in their habits the Dugongs 

 differ little from the Manatees, and they are often killed and 

 eaten. They attain a length of from eight to ten, twelve, or 

 more feet, and are found on the coasts of the Indian Ocean 

 and its islands, extending their range to the north coast of 

 Australia. The bones are remarkable for their extreme den- 

 sity, their texture being nearly as close as ivory. 



The Manatees and Dugongs, as before said, are the only 

 living Sirenia; but besides these there is a very singular form, 

 the Rhytina Stelleri, which is now extinct, having been exter- 

 minated by man within a comparatively recent period. This 

 remarkable animal was discovered about the middle of the 

 eighteenth century in a little island (Behring's Island) off the 

 coast of Kamtchatka. Upon this island the celebrated voyager 

 Behring was wrecked, and he found the place inhabited by 

 these enormous animals, which were subsequently described 

 by M. Steller, who formed one of his party. The discovery, 

 however, was fatal to the Rhytina, for the last appears to have 

 been seen in the year 1768. The Rhytina was an animal of 

 great size, measuring twenty-five to thirty-five feet in length, 

 and twenty feet at its greatest circumference. There can 

 hardly be said to have been any true teeth, but the jaws con- 

 tained -r large lamelliform fibrous structures, which offici- 

 ated as teeth, and may be looked upon as molars. These 

 singular structures are not teeth, in the true sense of this term ; 

 but they are similar to the horny tuberculated plates found in 

 the front of the mouth of the Dugong and Manatee, and the 

 upper ones may be regarded as the equivalent of the anterior 

 palatine pad of the Ruminants (Murie). The epidermis was 

 extremely thick and fibrous, and hairs appear to have been 

 wanting. There was a crescentic tail -fin, and the anterior 

 limbs alone were present. 



As regards the distribution in time of the Sirenia, the oldest- 

 known remains referable to the order are found in the Eocene 

 Tertiary (Eotherium). Of the same age is probably the inter- 

 esting form described from the Tertiary deposits of Jamaica by 

 Owen under the name of Prorastomus sirenoides. This type 



