488 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
In the scaldiceti the premaxille, maxille, and mandible still bear 
teeth. The same is true of a whale belonging to a later formation, 
Physeterula (fig. 17), in which, however, the enamel layer of the 
crown is already lost. 
Then follows a forerunner of the sperm whale, Prophyseter 
(fig. 18), in which the incisors fall out in early youth, while the 
maxillary teeth follow a little later. The upper dentition is, there- 
fore, gradually aborted. 
A. 
B. 
Fic. 16.—Skull of Scaldicetus patagonicus Lydekker from the Miocene of Chubut, Patago- 
nia. A, from in front; B, from the right side. 4 natural size. After R. Lydekker. 
Then follows the genus Placoziphius, in which the premaxille 
and maxille have become entirely toothless, while only the lower jaw 
bears teeth, and thus we reach the stage where the existing sperm 
whale (fig. 20) joins on. 
There are few genealogical series of animals which give us the 
history of a stock so clearly. Of special interest, however, is the 
sudden, almost “ explosive,’ development from the squalodons to the 
sperm whale. The entire development is completed in a very small 
