486 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
This last genus is a transition form to the later primitive ceta- 
ceans of the upper Middle Eocene. In this form, Zeuglodon (fig. 
14), the marked difference between the incisors, canines, and anterior 
premolars has disappeared; the posterior teeth have lost their three 
roots and have therefore become two-rooted. The last molars of the 
upper jaw have begun to retrograde, and in one species (fig. 14) are 
already lost. 
From the scanty remains of the limbs of this primitive cetacean 
which have been discovered, only one conclusion can be drawn, 
namely, that the arm was modified so as to form a flipper. As the 
tail vertebra of the later primitive forms are very large and power- 
ful and resemble those of the existing whales, it is certain that these 
creatures swam after the manner of the whales and not of the seals. 
Vie. 14.—Skull of a primitive cetacean, Zeuglodon osiris Dames, from the upper 
Middle Eocene of Egypt. Length of skull 70 cm. (27.5 in.). incisors ; 
C=canine; P=premolars; M=molars. After HE. vy, Stromer. 
The primitive cetaceans appear to have become extinct with the 
latest form of the Upper Eocene. At all events, no remains from 
later deposits give us any evidence of a continuation or transforma- 
tion of this stock. We are therefore confronted anew with a great 
question: Where shall we look for the ancestors of existing whales? 
A small primitive cetacean from the Eocene deposits of the Caucasus, 
Microzeuglodon, gives us a clew. This appears to be the progenitor 
of a series which is represented in the Oligocene only by a small 
whale from the vicinity of Biinde in Hanover, but which in the Mio- 
cene (the next later division of geological time) reached a high de- 
velopment. This Miocene whale has a very full dentition. The 
teeth remind one remotely of those of the sharks. It is, on this 
account, called the “shark-tooth whale,” Sqgualodon. (Fig. 15.) 
