64 CETACEA. 



dugony from Indian, and H. anstralis from Australian seas, but 

 no valid characters by which to distinguish the various forms 

 appear to have been adduced. 



Order II. -GET ACE A. 



Body fusiform, passing anteriorly into the head without any 

 distinct neck, posteriorly tapering off gradually to the base of the 

 tail, which is provided with a pair of lateral, pointed expansions 

 of skin, forming together a horizontally placed triangular propell- 

 ing organ, notched in the middle line behind. Head generally 

 large, sometimes more than one-third of the total length ; aperture 

 of mouth always wide, bounded by stiff immobile lips. Fore limbs 

 modified into flattened ovoid paddles, encased in a continuous 

 integument. No external signs of hind limbs. General surface 

 of skin smooth and glistening, devoid of hair. A compressed 

 median dorsal fin almost always present. Eye small ; no nictita- 

 ting membrane nor true lachrymal apparatus. Auditory opening 

 consisting of a minute aperture in the skin situated a short distance 

 behind the eye, without vestige of pinna. Nostrils opening 

 separately or by a single crescentic valvular aperture near the 

 vertex. Teeth generally present but very variable in number, of 

 simple, uniform character, having conical, compressed crowns, 

 and single roots, and never preceded by milk teeth. Among the 

 Mystacoceti the teeth are absent (seep. 65). No clavicles. Im- 

 mediately beneath the skin and intimately connected with it, is a 

 thick layer of fat, held together by a dense mesh of areolar tissue 

 constituting the blubber, which serves the purpose of the hairy 

 covering of other mammals in retaining the heat of the body. 



Cetaceans abound in all seas and in some of the larger rivers of 

 Asia, such as the Ganges, Indus, and Irrawaddy, and of South 

 America where the " Inia " of the natives of Bolivia ascends the 

 Amazon even to its remote sources among the Peruvian Lakes. 

 Necessarily from the structure of their limbs they are purely 

 aquatic mammals and once stranded, from any cause whatever, 

 are absolutely at the mercy of their smallest enemy. For the 

 purpose of respiration it is necessary that they should rise fre- 

 quently to the surface, and since this necessitates a frequent 

 upwardly rising motion and a subsequent plunging downwards 

 the tail is therefore expanded horizontally, not vertically as in 

 Fishes where no such motion is requisite. For an equally benefi- 

 cent reason the respiratory orifice is placed on the highest point, 

 or vertex, of the head, with the purpose of enabling the animal to 

 breathe without trouble while at the same time exposing the 

 smallest possible portion of its body above the surface, where it 

 might necessarily expect to meet with its most dangerous foes and 

 where its powers of vision, such as they are, would be hopelessly 



