~ 
HISTORY OF MARINE MAMMALS—ABEL. 489 
section of geological time, the Upper Miocene, and since that time 
the sperm whales have not materially changed. 
Fic. 17.—Skull of the physeterine species Physeterula dubusii Van Ben, from the Upper 
Miocene of Antwerp. The beak is lacking. j~ natural size. 
A second series springs from the squalodons and leads to the 
existing beaked whales. Here the simplification of the dentition is 
completed in a different way than in the case of the sperm whales. 
Fig. 18.—Anterior end of the beak of an ancestor of the Sperm whale, Prophyseter dolloi 
Abel, from the Upper Miocene of Antwerp. 18a, from below; 18b, from without. On 
the left the premaxilla, on the right the maxilla. 4 natural size. 
The most important difference is that all the teeth disappear, with 
the exception of one or two pairs in the lower jaw. In the living 
