PORPOISES AND DOLPHINS 319 



floating ice. The principal enemy of Commerson's Dolphin is 

 probably Orcinus, the Killer Whale." 



The stomach of one Commerson's Dolphin contained, as 

 well as the pens of cuttlefishes, shrimps similar to those which 

 form the food of the whalebone whales. 



Genus Lagcnorhynchus. 



The characters which most definitely separate Lageno- 

 rhynchus from nearly related genera are, unfortunately for our 

 present purpose, osteological ones such as the great number 

 of vertebrae (80 to 90), the great length of the transverse and 

 vertical bony processes from the vertebrae and features of the 

 skull which need not be mentioned here. Of distinguishing 

 external characters the following are noteworthy, namely, the 

 short, rather ill-marked beak, the moderately high pointed 

 back fin having concave posterior border, the prominent 

 dorsal and ventral ridges behind dorsal fin and vent and the 

 moderately-sized pointed flippers. 



THE WHITE-SIDED DOLPHIN {Lagenorhynchus acutus). 

 Fig. 86. 



This species, which presents all the external features men- 

 tioned above in the generic description, may be recognized 

 by the distribution of pigmentation on the body, but the 

 extreme development of the dorsal and ventral ridges on the 

 tail is worthy of notice. As its common name implies, there is 

 an area of light colour on each flank enclosed by the more 

 heavy pigmentation general to that region. The light area 

 forms a longitudinal band extending from below the dorsal 

 fin almost to the tail flukes. It is made up of two portions, 

 an upper part having little or no pigment and a lower yellowish 

 portion. The back is black from the tip of the snout to the 

 flukes. The under surface is mainly white, but shades into 

 grey forwards to the chin and in the region of the vent is a 

 small black area. The flukes and flippers are pigmented, and 

 from the latter a narrow black streak runs forward to the angle 

 of the mouth. 



The teeth number 30 to 34 pairs in upper and lower jaws ; 



