270 WHALES AND DOLPHINS 



Genus Hyperoodon. 

 THE BOTTLE-NOSED WHALES. Fig. 69. 



The genus Hyperoodon includes the two species, H. rostratus, 

 the northern Bottle-nosed Whale, and H. planifrons, its 

 congener in the southern hemisphere. Little is known about 

 the external form of the latter species, so that the description 

 of the genus given here refers particularly to the more abundant 

 northern form. The common Bottle-nosed Whale is, when 

 fully grown, of moderate size, males attaining to a length of 

 some 30 feet and females to about 24 feet. The front end of 

 the body is characterized by a snout more sharply defined 

 from the remainder of the head than in any of the other 

 Ziphioids, and by the presence of a prominent " forehead " 

 which in the male bulges more with age. Underlying the 

 " forehead ", and responsible for its change in form, are two 

 crests of bone, one on each side, on the forward extension or 

 rostral portion of the skull. These crests are low and widely 

 separated in the young Hyperoodon, but as the animals become 

 adult they grow more and more massive, approximating to 

 each other to form a boss of great compactness and solidity. 



From the head the outline of the body passes evenly back- 

 ward into that of the trunk, which has its greatest girth behind 

 the insertion of the flippers, from whence it tapers towards 

 the tail. The back fin is considerably behind the middle of 

 the body, and neither in form nor in position is it distinguishable 

 from that of any other Ziphioid. The flippers also, relatively 

 small and pointed, are without characteristic generic distinc- 

 tion. The single blowhole, as in all Ziphioids, is crescentic in 

 shape with the concave margin towards the snout. Grooves 

 are to be found in the skin in the region of the throat and two 

 seems to be the typical number, but in some embryos examined 

 by one cetologist four were to be seen. 



The general body colour is very dark grey to black on the 

 dorsal surface, and somewhat lighter grey to white on the 

 ventral surface. Flippers and flukes are dark coloured on both 

 upper and under surface. Old animals tend to be generally 

 lighter in colour than young ones. 



The similarity of appearance in all Ziphioids makes the form 



