504 MANUAL OF ZOOLOGY. 



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. 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 chiefly on the coasts of the Indian Ocean. The bones 

 are remarkable for their extreme density, 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 Kamschatka. 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 feet in length, and twenty feet 

 at its greatest circumference. There can hardly be said to 



have been any true teeth, but the jaws contained large 



lamelliform fibrous structures, which officiated as teeth, and 

 may be looked upon as molars. 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. 



ORDER V. CETACEA. In this order are the Whales, Dol- 

 phins, and Porpoises, all agreeing with the preceding in their 

 complete adaptation to an aquatic life (figs. 195, 196). The 

 body is completely fish-like in form ; the anterior limbs are con- 

 verted into swimming - paddles or "flippers"; the posterior 

 limbs are completely absent ; and there is a powerful, horizon- 

 tally-flattened, caudal fin, sometimes accompanied by a dorsal 

 fin as well. In all these characters the Cetacea agree with the 

 Sirenia, except in the one last mentioned. On the other hand, 

 the nostrils, which may be single or double, are always placed 

 at the top of the head, constituting the so-called " blow-holes " 

 or " spiracles " ; and they are never situated at the end of a 

 snout. The body is very sparingly furnished with hairs, or 

 the adult may be completely hairless. The testes are retained 



