HISTORY OF WHALES 



49 



skull of Xenorophus has a rather elongated 

 rostrum, it differs from the zeuglodont 

 skull in having the opening leading to the 

 nostrils located behind the level of the 

 anterior margin of the orbit. The nasal 

 bones are noticeably shortened, and their 

 bases are mortised into the frontals near 

 the level of the posterior margin of the 

 orbit. The palatine bones of this skull 

 extend forward beyond the antorbital 

 notches, and there is no evidence that 

 their position with respect to other 

 basicranial elements was affected to any 

 marked extent by the backward thrust 

 of the maxillary. The posterior extrem- 

 ity of each palatine abuts against the 

 corresponding edge of the pterygoid and, 

 unless they were permitted to slide under 

 the latter, they would remain in approxi- 

 mately their original positions, but a 

 telescoping of these elements has occurred 

 in some of the living porpoises, for in 

 Mesoplodon the palatines are largely over- 

 spread by the greatly enlarged pterygoids, 

 while in Platanista the palatines are 

 greatly reduced and are completely over- 

 spread by the pterygoids. 



The idea that the skull of the Upper 

 Eocene Agorophius pygmaeus (Leidy, 1869; 

 True, 1907) bore some resemblance to the 

 rorquals apparently originated with Paul 

 Gervais (1871), and Cope (1895) remarked 

 that if teeth were absent it would be 

 necessary to refer this cetacean to the 

 Mysticeti. The possibility that Agoro- 

 phius may be one of the direct ancestors 

 of the whalebone whales (Mysticeti) 

 appears to be excluded, in view of the 

 limitations imposed by the actual mechan- 

 ical construction of the skull. In Agoro- 

 phius the maxillary broadly overspreads 

 the supraorbital process of the frontal 

 and does not stop in front of or project 

 backward beneath it, as in all known 

 Mysticeti. On the contrary in skulls of 

 both fossil and living whalebone whales 



the proximal end of the maxillary as seen 

 from a dorsal view abuts against the 

 supraorbital process of the frontal for 

 most of its width, but is attached to the 

 frontal in the interorbital region by a 

 narrow ascending process. Once inaugu- 

 rated it is difficult to conceive how the 

 general process of telescoping could be 

 reversed, since the backward slippage 

 of the maxillary depends upon the removal 

 of an obstruction consisting of the infra- 

 orbital portion of this bone, a process 

 furthermore that is retained by all known 

 whalebone whale skulls. Any assump- 

 tion that the mysticete skull was derived 

 from an Agorophius type necessitates a 

 reversal of the impetus that led to the 

 backward thrust of the proximal rostral 

 elements in addition to a secondary de- 

 velopment of an interlocking maxillary. 

 To accomplish this the broad plate of the 

 maxillary which already has overspread 

 the supraorbital process must be reduced 

 to a narrow ascending process and a 

 broad infraorbital process must arise. 



The association of Agorophius with the 

 odontocetes rests on a much sounder basis, 

 for this archaic toothed whale undoubt- 

 edly represents a somewhat distantly 

 related precursor of the squalodonts, 

 which include a number of diverse types. 

 There is reason to believe that Agorophius 

 at least represents a morphological stage 

 through which the toothed whales may 

 have passed in their development and that 

 the subsequent remodeling of the odon- 

 tocete cranium was accomplished by a 

 forward movement of the occipital region 

 until the supraoccipital shield came in 

 contact with the frontals. Additional 

 support to the view that the parietals 

 were crowded out by some forward move- 

 ment of the posterior elements is to be 

 found in skulls of immature bottlenosed 

 porpoises (Tursiops truncatus), in which the 

 parietal bones form the outer upper border 



