180 IOWA GEOLOGICAL SURVEY 



inence close to the inner margin, which plays into a correspond- 

 ing depression of the upper pair. No positive evidence of the 

 occurrence of vomerine teeth has yet been detected. 



Our knowledge of Mylostoma is confined at present to three 

 species, all from the Cleveland shale of Ohio. These are, M. 

 variabile Newberry, which is typical of the genus ; M. terrelli 

 Newberry, founded upon a unique example of a lower dental 

 plate ; and M. newberryi Eastman, of which the complete lower 

 dentition is known. Notwithstanding the comparatively late 

 geological horizon of all these forms, they are not to be regarded 

 as incipient in the Upper Devonian, but as survivals of a 

 primitive type of Arthrodire in which the crushing type of Dip- 

 noan dentition was a persistent feature. 



One of the interesting points established by Professor Dean's 

 study of the type species of Mylostoma is the close agreement 

 between it and Dinichthys in all essential respects save for the 

 dentition; and as regards this latter feature, the same differ- 

 ence is to be noted as exists between Ehynchodus and Palaeo- 

 mylus among Ptyctodonts, or between Protopterus and Neocer- 

 atodus among modern Dipnoans. Parallel modifications of this 

 nature, occurring as they do in very diverse groups, are doubt- 

 less to be correlated with similar food habits. Among Chimae- 

 roids, for instance, certain genera are shown by their develop- 

 ment of tritoral dental plates to have subsisted on hard-shelled- 

 prey, such as mollusks, echinoderms and the like, thus meriting 

 Dollo's term of "conchifrage";* whereas others, as indicated 

 by their sharp sectorial rims, were adapted for subsistence on 

 soft tissues, and were probably predaceous creatures ("macro- 

 phage" Dollo). Mylostoma and Dinichthys furnish examples of 

 corresponding adaptations among Arthrodires, and an exact 

 parallel is found in modern Lung-fishes. 



The mandibles of Mylostoma would seem to have retained with 

 great persistency typical Dipnoan conditions. The form of the 

 dental plates strongly recalls the unmistakable Ceratodont con- 

 figuration, and these elements are more sharply demarcated 

 from the supporting splenial than in other Arthrodires. Al- 



*Dollo, L., Sur quelques points d'6thologie paleontologique. Bull. Soc. Beige 

 Geol. 1906, 20, p. 1. 



