134 Eastman — D/'jmoan Affinities of Arthrodires. 



logies between them and Neoceratodus. Provided a case be 

 made out, the structural resemblances between the two types 

 can hardly be explained except on the hypothesis of a common 

 origin. It therefore becomes necessary to suppose that a 

 generalized archetype of modern Dipnoans, probably derived 

 directly from the Elasmobranch stem, was present during the 

 lowermost Devonian, giving rise to two specialized groups, 

 but manifesting itself, like Limulus, Cestraeion, Scorpions and 

 other archaic survivals, extraordinary persistence and conserva- 

 tism ever since. The relations between the three recognized 

 orders of Dipnoans would then appear after some such scheme 

 as this : 



Neoceratodus 



Ceratodus 



sturii 



Ctenodus 

 Uronemus 

 Phaneropleuron 

 Scaumenacia 

 Dip terns 



Titanic hthys 

 Coccosteus, Dinichthys 

 Dinomylostoma, Mylostoma 

 Homosteus 

 Macropetal ichthya 



Primitive Ceratodonts 



In what follows we shall endeavor to show (1), that the 

 dentition of Arthrodires is distinctly of the Dipnoan type, aris- 

 ing in the same manner and representing the same elements ; 

 (2) the relations of the Meckelian cartilage are identical in 

 both types ; (3) the dermal plates forming the cranial roof have 

 undergone corresponding reduction and are arranged after 

 essentially the same pattern, both in Arthrodires and Cerato- 

 donts ; and (4), Neoceratodus recalls throughout its entire 

 organization, save for the absence of dermal armoring, the 

 principal features of Arthrodires. Such intimate structural 

 resemblances cannot be explained by parallelism, but point 

 plainly to common descent. 



Dentition of Dinichthys and Neoceratodus compared. — So 

 long ago as 1875, Newberry* was struck with the close par- 

 allelism between the jaws of Dinichthys and those of Pro- 



* Newberry, J. S., Eept. Geol. Surv. Ohio, Palseont. vol. ii (1875), p. 15. 



