IV. VERTEBRATA: MAMMALIA, CETACEA. 645 



ten molars; Halicore dugong, Indo-Pacific; Rhytina stelhri of the 

 northern Pacific, exterminated in 1768. 



Order IX. Cetacea. 



In external form the whales resemble the sirenians, a result of 

 an aquatic life, but the resemblance ends here. The whales are 



FIG. 668. Restoration of skeleton of HaUtherium, an extinct sirenian. (After Miss 



Woodward.) 



so fish-like that they are commonly included by the laity in that 

 group, and every one speaks of the whale fishery. Head and trunk 

 are scarcely distinguished, the cervical vertebrae being very short 

 and more or less completely fused. The hinder limbs are absent, 

 and of the pelvic girdle only a small ilium remains, and no sacral 

 vertebras are developed. The caudal fin is two-lobed and differs 

 from that of a fish in being horizontally flattened ; the skin is thick 

 and is sparsely haired or completely naked, in some hair being 

 lacking even in the embryo. Most of the species inhabit the high 

 seas, Inia botiviensis and Platanista gangetica occur in rivers. 



The fore limbs are modified into flippers, the bones of which are of 

 nearly equal size and are jointed only at the shoulder. A dorsal fin ( ' fin 

 backs ' ) occurs in some. The lack of hair is compensated by the thick 

 layer of subcutaneous fat (blubber) which, like the fat penetrating the 

 spongy bones, tends to lessen the specific gravity. In order that the ani- 

 mals may breathe while feeding, the larynx is prolonged into a tube 

 which extends up through the pharynx to the choanaB, from which the 

 nostrils extend directly upwards to the single (Denticetes) or paired (Mys- 

 ticetes) external opening. Since the air driven out with great force con- 

 tains much moisture and this is condensed on contact with the cooler 

 external air, the impression was natural that the animals in 'blowing' 

 spouted water. Since the olfactory membrane is degenerate and the 

 olfactory lobes are reduced, the nose is an organ of respiration only. 



The eyes are small, external ears are lacking, the mammas are close to 

 the sexual opening. Tho teeth are either present in large numbers, similar 

 and conical, and, since the second dentition is rudimentary, are mono- 

 phyodont (Denticetas) or they are outlined early and then resorbed and 

 replaced by plates of baleen (Mysticetaa). This is composed of large horny 

 plates (whalebone) in large animals a dozen feet long (fig. 669, ?/;), of 

 which several hundred are arranged in close succession extending inward 

 to the tongue. They correspond to the transverse palatal folds which 



