94 NEW YORK STATE MUSEUM 



saying that valuable enlightenment concerning the dentition of Arthrodires 

 is furnished by the type of a new genus from the Portage of western New 

 York, hereinafter described under the name of Dinomylostoma. This 

 interesting form, transitional between Dinichthys and Mylostoma, acquaints 

 us for the first time with the nature of the articulation between the lower 

 jaw and head shield amongst Coccostean fishes ; it displays portions of the 

 Meckelian cartilage, still preserved in natural position ; and the relations 

 of this structure to the ossified mandibular elements — splenial and dental 

 plate proper — are seen to be precisely as in modern Dipnoans. The lower 

 jaw of Arthrodires is therefore proven to conform strictly to the Dipnoan 

 type, a fact of cardinal importance for their classification. The upper den- 

 tition, also, is comparable in a general way to young stages of Neocera- 

 todus ; hence it may be affirmed with entire propriety that the dentition of 

 the recent genus passes through an early Mylostomid stage. Further com- 

 parisons between Neoceratodus and typical Arthrodires, such as Coccosteus, 

 Dinichthys and Mylostoma, establish a sound basis for the following 

 propositions : 



1 The dermal plates forming the cranial roof In Arthrodires and primi- 

 tive Ceratodonts have undergone corresponding reduction, and become 

 arranged after essentially the same pattern. 



2 Neoceratodus recalls throughout Its entire organization, save only 

 for the absence of dermal armoring, the principal features of Arthrodires ; 

 and no theory of parallelism is competent to explain the aggregate of these 

 resemblances. 



3 It is impossible to regard Neoceratodus as the degenerate descend- 

 ant of both Ctenodipterines and Arthrodires, nor of either of them singly, 

 since it partakes of the characters of both. Community of origin is there- 

 fore necessarily presupposed for all three groups, there being no alternative 

 explanation of their relations. 



4 Arthrodires and Ctenodipterines may be regarded as specialized 

 offshoots which diverged in different directions from the primitive Dipnoan 



