NO. 8 INTERRELATIONSHIPS OF THE CETACEA WINGE 23 



the face, is pressed by the water in against the bones and has moulded 

 the upper side of the whole face as its bed, especially hollowed out 

 posteriorly. This bed, the " facial depression," extends backward 

 along the sides of the nasals on the forehead. The beginning is 

 traceable of a remarkable peculiarity which, in the more advanced 

 cetaceans may become conspicuous to a high degree : an asymmetry 

 in the structure of the face. The head must no doubt be so held 

 during motion that the water comes to press not quite equally on both 

 sides, but more strongly on the right side than on the left. The facial 

 cushion therefore becomes larger on the right side than on the left, 

 extends its bed most on the right side, forces the nasal passage to 

 bend over to the left, and causes the bones of the face to develop 

 somewhat dissimilarly on the two sides. ''^ The maxillary has pushed 

 itself posteriorly up over the frontal to such an extent that it almost 

 wholly covers it, also spreading out over the supraorbital process. 

 The zygoma appears to have been quite slender. The two nasal aper- 

 tures were presumably united into one. In all of these points of 

 difference from the Balgenids the Squalodonts agree with the higher 

 cetacea, of whose most primitive forms they remind one in nearly 

 everything, so far as they are known, except in the condition of the 

 teeth. 



The Tertiary North American Agorophius, which is only known 

 from a very incomplete skull, almost without teeth, appears to be the 

 most primitive member of the family. The number of teeth is not 

 known, but certainly, to judge by the other peculiarities of the genus, 

 it must have been greater than typical. Its braincase is much less 

 telescoped than in the other Squalodonts, also less than in any of the 

 known Balsenids, somewhat suggesting the Zeuglodonts in being 

 relatively strongly constricted anteriorly between the large temporal 

 fossae, and in having the parietal form a considerable part of its roof. 

 In the other Squalodonts the braincase, so far as it is known, is so 

 telescoped and so broadened out to the sides that there is a wide area 

 between the temporal fossae although these are relatively large ; also 

 the parietal in the middle of the roof of the braincase shows itself at 

 most as a' narrow band. In other respects Agorophius appears to 

 agree well with Squalodon. 



Squalodon is known rather completely from skulls from Tertiary 

 strata in both the Old and New Worlds. Almost nothing is known 

 of other parts of the skeleton. The teeth are well developed, hetero- 

 dont. In each jaw there are three incisors with conical crown and 

 single root, a canine of similar form and size, and ii, or sometimes 



