NO. 8 INTERRELATIONSHIPS OF THE CETACEA WINGE 73 



thought that was not at the time carried further. But in 1908 Kiiken- 

 thal tried to give a more exact explanation. To begin with he thought 

 he could prove that the asymmetry in the bones of the face is found 

 not only in toothed cetacea, but also in the whalebone whales, though 

 only slightly defined. In two skulls of Balmioptera (of B. rostrata 

 and B. musculus) he had found certain of the facial bones just a 

 trifle broader on the right side than on the left. Next he discovered 

 that some embryos of toothed cetaceans, of Platanista, Stcno, Glo- 

 biccps, Delph'mus, Phocccna, Hyperoodon, had the caudal fin set 

 awry, not level but in such a position that the left fluke " etwas schrag 

 nach aufwarts, der rechte schrag nach abwarts gerichtet war " 

 (p. 614). In 12 embryos of Delphinapterus the fin was, on the con- 

 trary, horizontal. All the embryos of whalebone whales examined 

 {oi Balccnoptcra musculus and B. gigas) had the fin oblique in the 

 same manner as the toothed whales. How the fin is in adult cetaceans 

 is said to be not clearly understood ; a few observations by other 

 investigators may, however, indicate that the obliquity is present in 

 the adults also. When a whale propels itself forward by means of a 

 sculling movement of the oblique caudal fin it is said to turn at the 

 same time to the left : " Der Wal durchschneidet also bei derartiger 

 schrager Bewegung der Schwanzflosse das Wasser nicht genau in der 

 Richtung seiner Langsachse, sondern sein Weg verlauft von dieser 

 Geraden etwas schrag nach links zu " (p. 616). And from this is 

 said to result an oblique pressure of the water on the head, and con- 

 sequently the obliquity of the skull, since the bones on the left side 

 are pressed upon more than those of the right side, are made thicker, 

 etc. : " Der Druck der beim Schwimmen durchschnittenen Wasser- 

 massen wird auf die linke Seite des Vorderkopfes starker wirken als 

 auf die rechte. Dieser Druck pflanzt sich durch die elastischen 

 Weichteile des Vorderkopfes hindurch auf die darunter liegenden 

 Schadelknochen fort. Die Wirkung dieses starkeren Druckes muss 

 sich zunachst in einer Verdickung der entsprechenden Schadel- 

 knochen aussern . . . .'' (pp. 616-617). That the bones in the left 

 side of the face are not so wide as those of the right side is said to be 

 connected with the fact that the bones of the left side are the thickest : 

 " Es wird dadurch links eine kleinere Flache als rechts geschaffen, 

 welche den etwas starkeren Druck auszuhalten hat und damit bis zu 

 einem gewissen Grade einen Ausgleich gegeniiber der rechten Seite 

 herbeifiihrt " (p. 617). 



Abel believes that the reason for the asymmetry of the skull in 

 toothed whales is to be found in the atrophy of the nasal bones, etc.. 



