316 OUTLINES OF EVOLUTIONARY BIOLOGY 



fish-like ancestors. 1 The fore limbs have become converted into 

 paddles while the hind limbs have entirely disappeared externally 



FIG. 160. Skull of Dinotherium giganteum, Lower Pliocene, X T ^. (From 

 Smith Woodward's " Vertebrate Palaeontology," after Kaup.) 



(Fig. 161). The tail has become flattened out into a horizontal 

 fin and there is frequently a well developed dorsal fin. The skull 



FIG. 161. The Dolphin, Delpliinus delphis, X 



Guide.) 



(From British Museum 



has undergone very curious changes. The brain case is rounded 

 and strongly arched (Fig. 162) and the nasal apertures or blow- 

 holes lie far back, at or near the highest point of the head 



1 Compare Chapter XVII. 



