208 IOWA GEOLOGICAL SURVEY 



Pander, Huxley, Traquair and others, and particularly within 

 the last decade or so its relations to modern Lung-fishes have 

 engaged profound attention on the part of zoologists and palae- 

 ichthyologists. Recent discussion has focussed upon two rival 

 theories, as already noted. According to the first, which has 

 steadily gained in ascendency, the members of this order are 

 supposed to have attained greater specialization during the 

 Devonian and Carboniferous than all other Lung-fishes, and 

 existing members of the subclass are regarded as decidedly 

 more primitive in organization than Ctenodipterines. Modern 

 Sirenoids (i. e., Neoceratodus, Protopterus and Lepidosiren) 

 would therefore be looked upon as survivals of a more general- 

 ized, more archaic Dipnoan stock than Dipterus and its allies, 

 rather than as actual descendants of the latter. 



The second and later interpretation is that of Dollo,* and the 

 exact opposite of ( the first. Evidence drawn from other than 

 cranial characters is held to confirm the belief that Dipterus 

 itself is the most archaic of all Dipnoans, and that modern Lung- 

 fishes have been derived from it through successive stages of 

 specialization. As a starting-point for his theory, Dollo accepts 

 the conclusion previously reached by Balfour and Parker that 

 the apparently diphycercal tail of recent forms is secondary, 

 due to abortion of the termination of the vertebral axis, and 

 coalescence of the median fins. The less ossified condition of 

 the skull and chondrocranium in modern forms is also ex- 

 plained as due to secondary reversion, through degeneration, 

 to an apparently primitive condition. The chief objection to 

 Dollo 's view is that we are unacquainted with any parallel ex- 

 ample among vertebrates which justifies belief in the possi- 

 bility of such degeneration as is here assumed.! 



* Dollo, L., Sur la phylogenie des Dipneustes. Bull. Soc. Beige Geol. etc., 1895, 

 pp. 79-128. 



t For critical remarks on this point see Dr. Traquair's Vice-Presidential Address, 

 Rept. Brit. Assoc. Adv. Sci., Bradford Meeting, 1900, p. 776 et seq., and Professor 

 Bridge's Monograph on Lepidosiren, in Trans. Zool. Soc. London, 1898, 14, pp. 

 366-372. Still more recently Karl Fiirbringer has remarked upon the same subject 

 as follows (Jena Denkschr. 1904, 4, p. 498 et seq.): 



Ich kann dieser Ansicht [Dollo's] gegemiber, die sich namentlich auf die zeit- 

 liche Verbreitung der Dipnoer stutzt, nur betonen, dass am Schiidel der recenten 

 Dipnoer kein Anhalt dafiir besteht, dass hier Knochen verloren gegangen seien. 

 Wir sehen im Gegentheil solche in Bildung und im Wachsen begriffen. . . . 



