The Structure of Dmkhthys 211 



3. Possibly many of the Wildungen Arthrodira can also be placed in this order, 

 making a number of new families. 



The classification of the other Arthrodira cannot be satisfactorily accomplished at 

 the present; they are too incompletely known. We may merely indicate some groups. 



Mylostoma and Dinoynylostoma, together with the allied forms, should certainly 

 compose a separate order, Mylostomida. 



Titanichthys must be accorded the same treatment. 



Homostius and Heterostms, which, according to the present writer's latest investiga- 

 tions, are some of the most highly specialized Arthrodires known, should very 

 probably be considered as representatives of two independent orders. 



Coming back to the first family of the order Coccosteida, we may give the following 

 definition : 



Family Coccosteidae 



Arthrodira with armor composed of relatively thick, solid plates. The eyes were 

 small, the sclerotic rings are broad. The sub-orbital is large, the post-sub-orbital small. 

 The gnathal elements are relatively short, broad and thick. The side plates of the head 

 are well separated from the head roof. 



Genus Coccosteus Ag. 



Relatively small forms, with tuberculate armor plates. The infero-gnathal is with- 

 out the large front tooth, but with well developed symphysial teeth. Median-dorsal 

 with a medium sized keel, long, running into a point. On its surface, impressions of the 

 sensory canals. Two sensory canals on the antero-dorso-lateral and on the antero-ventro- 

 lateral. The spinal in the form of a small spine-like process. 



Genus Dinichthys Nwb. 



Relatively large forms, with almost non-tuberculate armor plates. The infero- 

 gnathal with a well developed front tooth. The symphysial teeth if developed, small. 

 Median-dorsal with large and massive keel, short and roundish. No sensory canals on 

 MD and AVL. A single canal on ADL. The spinal is in the form of a small curved 

 plate. 



As seen from the following table, there have been described in all, 27 different species 

 of Dinichthys. Among them only a few are really well known. The majority have as a 

 type specimen a fragment of jaws or of the armor plates. To make a satisfactory defini- 

 tion of all these forms, a revision of all the old material and descriptions would be neces- 

 sary. This would require a long and careful study, which is not the object of this paper. 



