164 



IOWA GEOLOGICAL SURVEY 



sents no indications of having had the abdominal region pro- 

 tected by a system of dermal plates. We may assume, provis- 

 ionally, that this Meso-Devonian genus does indeed represent the 

 most primitive known stage of evolution among Arthrodires ; and 

 in this case we may fix upon Homosteus as occupying an inter- 

 mediate position between it and typical Coccosteans. These lat- 

 ter forms represent the terminal members of a divergent series 

 that agree with Mylostomids as regards cranial pattern and ar- 

 rangement of body armoring, but differ from them in respect to 

 the dentition. A graphical representation of these stages is 

 attempted in the following scheme, with what claims to verisim- 

 ilitude we leave others to judge. It will at least serve to empha- 

 size the point that in any conjectural line of descent two diverg- 

 ent series of Arthrodires must be recognized, agreeing more 

 or less as regards cranial pattern, but differing with respect 

 to the dentition. 



Dinichthys, Titan- 

 ichthys, etc. 



Coccosteus 



Homosteus 



Mylostoma 



Dinomy- 

 lostoma 



(Unknown 

 primitive 

 stage) 



Macropetalichthys 



Ancestral Ceratodont stock 

 (Generalized Dipnoans) 



