14 PALEONTOLOGY OF OHIO. 



tms and DinichtJiys are the same; the only difference being that in 

 Heterostius this bone is mnch shorter than broad, while in Dinichthys 

 the two diameters are nearly equal. 



The cranium of Dinichthys seems also to approach more nearly in 

 structure to those of Asterolepis and Heterostius than to that of Gocgos- 

 teus. The resemblance would also seem to be somewhat closer with 

 Heterostius than with Asterolepis. In the latter genus the occipital por- 

 tion of the cranium forms nearly a straight, transverse line, while in 

 Heterostius it is very much arched, as in Dinichthys. The posterior 

 margin of the supra-occipital plate — which forms the center of the arcli — 

 has nearly the same character in Dinichthys and Heterostius, that is, it is 

 impressed with two deep pits separated by a narrow I'idge, and behind 

 these is a peculiar pyramidal projection. All the bones of the hinder 

 part of the head in Dinichthys are usually found soldered firmly together, 

 the union between the Supra-occipital and the Epiotics being so firm that 

 their points of junction cannot be discerned, and they seem to form one 

 solid bone. This bone is, however, sometimes found disarticulated, and 

 it is figured on Plate LIX. 



The anterior portion of the head is generally dismembered, and seems 

 to have consisted of strong bony plates lined wiUi and more or less fii-mlv 

 united by cartilage, as in Heterostius and Asterolepis. 



The outer surfaces of the external bones of Dinichthys are marked with 

 a fine granular, almost imperceptible ornamentation. In addition to this, 

 the cranial surface is inscribed with a series of excavated lines which form 

 an ornamental pattern, of which the outlines have not yet been fully 

 made out. Something of the sort is discernible on the crania of nearly all 

 the Placoderms. It may also be seen on the Post-Temporals (" Supra- 

 Scapulas "), and in Dinichthys and Coccosteus traces of it are visible on the 

 bones of the plastron. 



There is this marked difference, howevei", between Dinichthys and its 

 congeners, Asterolepis, Heterostius, Coccosteus, Pterichthys, Aspidich- 

 thys, etc., that they all have the surfaces of their external plates orna- 

 mented with thickly-set and often prominent and stellate tubercles, while 

 the surface bones of Dinichthys show only the granulation and linear fur- 

 rows referred to above. 



In one instance the anterior part of the head has been found entire, but 

 this was in a concretion, and it was so much injured by the removal of its 

 hard and tough matrix, that the outlines of the plates composing it can- 

 not be discerned. This shows, however, that a sheet of bone covered the 

 entire surface of the head. In all the crania found at Sheffield, the muz- 

 zle has disappeared ; only the occipital and central portions remaining. 

 The brain-box was apparently partly bone and part cartilage ; as we find, 



