XIII 



PHYLUM CHORDATA 



571 



mainly m the possession of rooted heterodont teeth, and in the 

 position of the narial aperture, which is situated comparatively far 

 forwards : the limbs are not known. The remains of both Whale- 



FIG. 1164. Mylo don robustus. (Restoration, after Owen.) 



bone Whales and Toothed Whales occur abundantly in Pliocene 

 deposits, some belonging to extinct, others to existing, genera. 

 Toothed Whales occur also in Miocene formations, and there, as well 

 as in the Pliocene and Pleistocene of Europe, North America, New 



FIG. 1165. Squalodon. Three of the lower true molars. (After Flower.) 



Zealand, and Australia, are represented by an extinct family, the 

 Squalaofowtidce (Fig. 1165), with heterodont dentition. 



The order Sirenia is first met with in the Eocene, and was repre- 

 sented in that and succeeding periods by several genera which 

 have become extinct, composing a family- -the Halitlieriidccof 



