

HISTORY OF WHALES 



47 



close of the Oligocene they had attained a 

 high degree of specialization. The char- 

 acters retained, especially in the skull and 

 teeth, are primitive and not sufficiently 

 diagnostic to connect them with any 

 known species of zeuglodont, and the 

 telescoping or remodeling of the skull is 

 already so far advanced that the incipient 

 stages of this general process are obscured 

 by certain details of cranial construction. 



One may visualize the steps in the 

 transformation of a zeuglodont type of 

 skull to that of the Eocene archaic toothed 

 whale, but skulls demonstrating the actual 

 stages have never been found. Assuming 

 that the skulls of these archaic toothed 

 whales have been derived from the zeu- 

 glodont type of skull, one must necessarily 

 concede that the outer edge of the max- 

 illary immediately in front of the supra- 

 orbital process of the frontal and the 

 tooth-bearing portion of the maxillary, 

 which projects backward beside the jugal 

 below the orbit, must degenerate or be 

 eaten away in order that the maxillary 

 may be pushed back over the supraorbital 

 process of the frontal. The jugal must 

 lose its connexion with the maxillary and 

 fuse to the ventral surface of the lachrymal 

 bone. The lachrymal bone must continue 

 to retain its position at the anterior margin 

 of the supraorbital process of the frontal, 

 but must be shifted so that it comes to lie 

 on the ventral instead of the lateral face 

 of the maxillary. 



AGOROPHIIDAE 



Skulls of the Upper Eocene genera 

 Xenorophus and Agorophius conclusively 

 show that the maxillary and premaxillary 

 have been subjected to a backward thrust, 

 which forced these bones over the supra- 

 orbital process of the frontal and carried 

 the antorbital foramen backward until 

 it came to lie near the level of the antor- 

 bital notch. These skulls, however, do 



not give us any clew as to how this 

 slipping was accomplished. By referring 

 to text figure 4 one can see at a glance in 

 what respects the backward thrust of the 

 proximal end of the rostrum has changed 

 the relative positions of certain elements. 

 In all known zeuglodont skulls the maxil- 

 lary straddles the anterior face of the 

 supraorbital process of the frontal in a 

 manner comparable to that existing in the 

 living whalebone whales (Mysticeti). 

 Unless the infraorbital portion of the 

 maxillary atrophied and finally disap- 

 peared with the reduction of the molar 

 teeth, and we have no direct evidence to 

 show that such was the case, no backward 

 movement of the maxillary would be 

 permitted. 



In the case of Xenorophus sloanii (Kellogg, 

 19x3), the maxillary is excluded from the 

 orbital region by the combined jugal and 

 lachrymal. The lachrymal bone has al- 

 ready assumed a position typical of 

 toothed whales (Odontoceti), abutting 

 against the anterior border of the supra- 

 orbital process of the frontal and mortised 

 into the ventral face of the maxillary. 

 Nevertheless, this bone differs from the 

 lachrymal in all known porpoises in hav- 

 ing an ascending process, which has 

 overspread the outer upper border of the 

 supraorbital process of the frontal. The 

 remodeling of the skull has proceeded in 

 directions that have not been followed by 

 odontocetes of later geological stages. 

 Some inexplicable peculiarity in the skull 

 of an immediate precursor gave rise to 

 unusual conditions in the Xenorophus skull, 

 which differs from all other known 

 cetaceans in having the premaxillary 

 widened posteriorly so that it extends 

 conspicuously outward underneath the 

 maxillary over the basal half of the 

 supraorbital process. Furthermore, the 

 maxillary slopes very abruptly in front of 

 the antorbital notches. Although the 



