DEVONIC FISHES OF THE NEW YORK FORMATIONS 



107 



^PtO 



plates here called centrals are correctly identified as such is evident from 

 the following reasons : first, the two pairs together occupy the usual posi- 

 tion of the centrals with reference to the preorbital 

 plate in front, and to the postorbital and marginal 

 plates externally. Secondly, 

 they are proved to be such 

 by the disposition of sen- 

 sory canals. Imagining the 

 suture line as obliterated 

 between the two independ- 

 ently ossified plates on 

 either side, we shall have 

 a single pair of elements 

 occupying the same rela- 



• TT ^ 



tive position as in Homo- 



SteUS. The posterior llZrlt, I/=margin7lT?olpreorbit"l'^ 



r . . , . /'/0=postorb;taI. Original in American 



moiety 01 this plate is seen Museum. Nat. size 



to be traversed by three different canal systems, 

 identifiable with those called by Dean the preorbital, 

 postorbital and occipital. Now, in the head shield of 

 all Arthrodires so far as known, the central is the 

 only plate traversed by all three of these canals. The 

 preorbital and postorbital systems are sometimes con- 

 fluent in other forms, but the occipital (or more prop- 

 erly, the exoccipito-central) does not unite with these 

 other systems save only in Macropetalichthys. A 

 Fig. 21 Detached median occipital detached speclmen of the median occipital show- 



plate of Macropetalichthys showing 



sensory canals and partially de- inp" conflucnt caual systems IS represented in text 



nuded ornamentation. Original in ^ ^ '' 



Mus. Comp.Zool. Nat. size figUre2I. 



Evidence of the primitive nature of this genus, as compared with other 

 Arthrodires, is furnished by the following characters, which point to a sur- 

 vival of embryonic or ancestral conditions: (i) continuity of the sensory 



