Eastman — Dipnoan Affinities of Arthrodires. 131 



Art. IX. — Dipnoan Affinities of Arthrodires ; by C. R. 



Eastman. 



In the modern fauna, Neoceratodus stands out as an isolated 

 landmark which has preserved faithful indications of the 

 course evolution has taken amongst Dipnoan fishes. Compared 

 with its nearest surviving relatives, Protopterus and Lepidosi- 

 ren, it represents a relatively early larval stage of development ; 

 and its generalized organization bears witness to an extremely 

 ancient origin. Regarding the Ceratodont type as decidedly 

 more primitive in structure than that of Dipterus and its 

 allies, and this view is supported by weighty evidence, two 

 conclusions are possible with respect to their genetic relations. 

 Either the more primitive type was in existence as early as the 

 Devonian, and has survived practically unchanged ever since ; 

 or else modern Lung-fishes are to be looked upon as degen- 

 erate descendants of the Dipterus stock. 



Objections stand in the way of either theory. Opposed to 

 the first is the failure of Palaeontology to realize our concep- 

 tion of a 'Palwoceratodus'' — that is to say, of fossil organisms 

 standing in ancestral relations to both Ctenodipterines and 

 Ceratodonts ; and added to this is the difficulty of supposing any 

 primitive type to have come down tons from remote ages with- 

 out undergoing extensive modifications. The newer interpre- 

 tation, proposed some years ago by Dollo,* is attended with 

 still graver difficulties. For if we make Dipterus the initial 

 term of a series leading through various Palaeozoic genera and 

 culminating in modern Lung-fishes, it will be necessary, as 

 Professor Bridge has observed,f "to assume the possibility of 

 an ossified skull so far degenerating as to lose almost all trace 

 of endochondrial ossification, and secondarily revert to the con- 

 dition of a skull so completely cartilaginous, and so primitive 

 in other respects, as that exhibited by the living Ccratodus. 

 So far as I am aware, there is no evidence to justify belief in 

 such a possibility." 



Comparison of other structural features besides the skull 

 leads to altogether similar conclusions. Thus, the argument 

 that the heterocercal tail of Dipterus must be antecedent to 

 the diphycercal (or gephyrocercal) of Neoceratodus is met by 

 Fiirbringer^: in following wise : "J^achdem in jeder Beziehung 



*Dollo, L. , Sur la phyloge"nie des Dipneustes, Bull. Soc. Beige Geol., vol. 

 ix (1895), pp. 79-128. 



f Bridge, T. W., Morphology of the Skull in Lepidosiren, etc., Trans. 

 Zool. London, vol. xiv (1898), p. 370. 



X Fiirbringer, K., Beitrage zur Morphologie des Sbeletes der Dipnoer. 

 (Semon's Zool. Forschungsreisen in Australien, etc.), Jena Denkschr., vol. 

 iv (1904), p. 500. 



