158 NEW YORK STATE MUSEUM 



supposed to have attained greater specialization than any of their known 

 descendants in later times, those existing at the present day showing 

 decidedly more primitive characters as compared with Ctenodipterines. 

 Modern forms would therefore be looked upon as survivals of the more 

 generalized primeval Dipnoan stock, rather than as the direct descendants 

 of Dipterus and its allies. 



The other and newer interpretation is that of Dollo, and directly the 

 reverse of the first. Evidence drawn from other than cranial characters is 

 held to confirm the belief that Dipterus is the most archaic of all Dipnoans, 

 and that modern lung fishes have been derived through successive stages of 

 specialization. As starting point of this theory, Dollo accepts the con- 

 clusion 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 in modern forms is likewise explained as 

 the result of secondary reversion, through degeneration, to an apparently 

 primitive condition. The chief objection to this view is that we are unac- 

 quainted with any parallel example amongst vertebrates which justifies 

 belief in the possibility of such degeneration as is here assumed. 



Yet another view of Dipnoan relationships, or perhaps rather to be 

 considered as a modification of the first, is that already outlined in the 

 present memoir. The Ceratodont type is regarded as decidedly more 

 primitive in structure than that of Dipterus and its allies ; and either it or 

 its direct prototype is assumed to have been in existence at least as early 

 as the lowermost Devonic, to have given rise to the highly specialized 

 orders of Ctenodipterines and Arthrodires, and to have persisted practically 

 unchanged ever since. Neoceratodus therefore falls within the same cate- 

 gory as scorpions, king-crabs, Lingula, Cestracion and other archaic sur- 

 vivals which have manifested extraordinary persistence and conservatism 

 throughout nearly the whole sequence of geological formations. This view 

 may be said to rest almost entirely upon the evidence of comparative 

 anatomy, and has received as yet no confirmation through the discovery of 



