DEVONIAN FISHES OF IOWA 163 



of dental plates, whether we find it already in vogue during the 

 Devonian, or developed independently from the main stock in 

 the Permian,* or again in evidence at the present day, merely 

 as a derivative of the more simple, earlier, or perhaps even orig- 

 inal type — the crushing or triturating type — which apparently 

 resulted from the concresence and fusion of hardened shagreen 

 granules. The Mylostomid type of dental plate is, therefore, as 

 implied by its structure, of more primitive nature than the Coc- 

 costean or Dinichthyid type, and more faithfully reproduces the 

 ancestral condition of things. On the basis of the dentition 

 alone members of the latter category signalize themselves as 

 more highly modified than the former. But other parts of the 

 organization and the very similar structure of the headshield 

 and body armoring in both Mylostomids and Coccosteids prove 

 that these families have become relatively further advanced in 

 certain directions than contemporary or earlier families (those 

 typified by Macropetalichthys and Homosteus, for instance) of 

 whose dentition we know little or nothing. In a word, the evi- 

 dence of dental characters is decisive and positive enough so far 

 as it goes, but should be supplemented by our knowledge of 

 associated skeletal details before determining the precise rank 

 of different families and genera. Present information, however, 

 regarding these concomitant characters is in many cases meagre 

 or very deficient. 



Under these circumstances it is evident that data are want- 

 ing for a detailed classification of Arthrodires, and even a genetic 

 grouping can only be provisionally outlined. As already inti- 

 mated, the general indications (apart from dental characters) 

 are that Macropetalichthys and Homosteus represent earlier and 

 less advanced stages of evolution than the grade of typical Coc- 

 costeus-like forms. In case the former of these genera were 

 eventually found to possess the triturating type of dentition, it 

 would fulfil very satisfactorily the requirements of an exceed- 

 ingly primitive phase of Arthrodires, approximating closely to 

 the primal Dipnoan stem. It is even conceivable that body 

 armoring is absent from this phase, in consonance with its 

 primitive condition; and certain it is that the genus itself pre- 



*The reference is to Sagenodus pertenuis, from the Permian of Texas and Russia 

 described in Amer. Nat. 1903, 37, p. 493. 



