CETACEA. 579 



singular animal is characterised by the great length of its 

 slender muzzle, and by the small size of the eyes. It attains 

 the length of seven feet, and the blow-hole is a longitudinal 

 fissure, and therefore quite unlike that of the typical Del- 

 phinida. The other fresh-water form is the Inia Boliviensis, 

 which inhabits the rivers of Bolivia, and is found at a distance 

 of more than two thousand miles from the sea. In its essential 

 characters it differs little from its marine brethren, and it 

 attains a length of from seven (female) to fourteen feet (male). 



The last of the Delphinidce. is the extraordinary Narwhal 

 or Sea-unicorn (Monodon monoceros). The Narwhal is an in- 

 habitant of the Arctic seas, and attains a length of as much as 

 fifteen feet, counting in the body alone. The dentition, how- 

 ever, is what constitutes the great peculiarity of the Narwhal. 

 The lower jaw is altogether destitute of teeth, and the upper 

 jaw in the females also exhibits no teeth externally, as a general 

 rule at any rate, though there are two rudimentary incisors 

 which do not cut the gum. In the males, the lower jaw is 

 likewise edentulous, but the upper jaw is furnished with two 

 molar teeth concealed in the gum, and with two incisors. Of 

 these two upper incisors, that of the right side is generally 

 rudimentary, and is concealed from view. The left upper 

 incisor, on the other hand, is developed from a permanent 

 pulp, and grows to an enormous size, continuing to increase 

 in length throughout the life of the animal. It forms a tusk of 

 from eight to ten feet in length, and it has its entire surface 

 spirally twisted. As an abnormality, both the upper incisors 

 may be developed in this way so as to form projecting tusks ; 

 and it is stated that the tusk is occasionally present in the female. 

 The function of this extraordinary tooth is doubtless offensive. 



Fam. 4. Rhynchoceti. This family is allied to the Cachalots 

 or Sperm-whales, and includes the so-called "Ziphioid Whales." 

 They are distinguished by the possession of a pointed snout 

 (the "beak" or "rostrum"), single blow-hole, small dorsal 

 fin and dentition. The upper jaw is edentulous, any teeth 

 which may be present not cutting the gum. The lower jaw, 

 on the other hand, possesses usually a single pair of teeth, 

 which are sometimes tusk-like, but which in other cases are 

 concealed by the gum. 



The rostrum of these Cetaceans is of great density, and has 

 often been preserved in a fossil state, usually presenting itself 

 of a bony cylinder or elongated cone, generally more or less 

 water-worn. The most important living genera are Hyperoodon 

 and Ziphius, of which the former is found in the North Atlantic, 

 and the latter in the Mediterranean and South Atlantic. 



