NO. 8 INTERRELATIONSHIPS OF THE CETACEA WINGE 55 



by the fusion of single teeth was already supposed by Eschricht 

 (I. c, p. 312). Should it be the case that there has taken place in 

 the whalebone whales a splitting up of serrate teeth, the correspond- 

 ing ancestral forms must probably have had teeth in which the cusps 

 on the fore a.id hind margins of the crown had great independence 

 and a size very nearly the same as in the principal cusp. In that 

 event it would not be easy to regard Patriocetus as an ancestor ; since 

 in it the cusps on the fore and hind margins particularly are weak in 

 proportion to the main cusp, and apparently in course of atrophy. 



Abel gives the following as his conception of the manner in which 

 the many small teeth of the Odontoceti, or at least of the Physeterids, 

 have arisen: " Dieser Spezialisationsweg des Gebisses (in the whale- 

 bone whales) ist fundamental von jenem verschieden, den wir in der 

 Phylogenese des Physeteridengebisses finden. Wie ich 1905 gezeigt 

 habe, tritt auf dem Wege zur Entstehung der Squalodontiden 

 zuniichst eine starke Vermehrung der mehrwurzeligen, vorn und 

 hinten gezackten Backenzahne ein, so dass sich das primitive Arch- 

 aeocetengebiss durch Vermehrung der Backenzahne im Pramolaren- 

 abschnitt zu dem polyodonten Squalodontidengebiss umformt. Aus 

 den Squalodontiden sind die Physeteriden hervorgegangen, bei 

 welchen das Gebiss eine Reduktion erf ahrt ; dieser Spezialisationsweg 

 fiihrt aber zu einer Vereinfachung der Krone, Verschmelzung der 

 bifiden Wurzeln, Reduktion der Zackenreihen am Vorder- und Hin- 

 terrande der Kronen zu einer krenelierten Leiste und endlich zum 

 ganzlichen Verlust der Schmelzkappen " (/. c, p. 187). Here Abel is 

 no doubt right in the main. It can only be objected that it cannot 

 exactly be said that Abel in his more special account (Die phylo- 

 genetische Entwicklung des Cetaceengebisses und die systematische 

 Stellung der Physeteriden ; Verhandl. Deutsch. Zool. Gesellsch., 1905, 

 pp. 84-96, and Les Odontocetes du Bolderien ; Mem. Mus. Roy. 

 d'Hist. Nat. de Belgique, vol. 3, 1905) has demonstrated that it is 

 precisely in the premolar region that the number of teeth has been 

 increased in the Squalodonts ; neither is it probable that the Physe- 

 terids originated directly from the Squalodonts. They appear to 

 have branched off at a higher level ; probably they had their root in 

 common with the Delphinids. 



There is no reason at the present stage for believing that the 

 increase in the number of teeth beyond the typical formula should 

 have had a different origin in the Mystacoceti and Odontoceti. In 

 view of the great resemblances that are everywhere found between 

 the two groups it is not likely that in this respect there would be a 



