318 OUTLINES OF EVOLUTIONARY BIOLOGY 



jaws of the porpoises and dolphins (Fig. 162) and in the lower 

 jaw only of the sperm whale. 



In the extinct shark-toothed Dolphins (Squalodontidae), whose 

 remains have been found in Miocene formations of Europe and 

 America, the teeth are still differentiated into incisors, canines, 

 prernolars and molars, and the molars have double roots and 

 compressed crowns with serrated edges. 



Further back, in Eocene times, there existed, widely distributed 

 over the northern hemisphere, a group of still more primitive 

 whale-like animals known as Zeuglodontidse. In these the seven 

 vertebrae of the neck, which in existing whales are more or 

 less fused together into a solid mass (Fig. 103), are all separate, 

 and the typical dental formula is identical with that of primitive 



,..31 4 32 



carnivorous land mammals, viz. i , c. - r , p.m. -., m. . 



o 1 4 o 



The genus Prozeuglodon, from the Egyptian Eocene, ap- 

 proaches so closely in the characters of the skull and teeth 

 (Fig. 163) to the primitive carnivores (Creodontia)of about the same 

 period as to leave no reasonable doubt about the derivation of the 

 Cetacea from that group, although it is quite possible that none 

 of the extinct forms so far discovered are actually in the direct 

 line of descent of any of the modern whales. 



