172 



IOWA GEOLOGICAL SURVEY 



Fig. 24. 



Fig. 24. Macropetalichthys rapheidolabis Norw. & Owen. Middle Devonian: Indiana, 

 Ohio and New York. Restoration of headshield showing arrangement of cranial plates and 

 course of sensory canals. ii 



C 1 , C 2 , divided centrals; EO, external occipital; M, marginal; HO, median occipital; P, 

 combined pineal and rostral, corresponding to the anterior median unpaired plate 

 ("mesethmoid") in Neoceratodus; PO, preorbital; PtO, postorbital plate. 



a single pair of elements occupying the same relative position as 

 in Homosteus. The posterior moiety of this plate is seen to be 

 traversed by three distinct canal systems, identifiable with those 

 called by Dean the preorbital, postorbital and occipital. Now, in 

 the headshield of all Arthrodires so far as known, the central 



