28 PALiEONTOLOGY OF OHIO. 



Ora/nmm. — The form and dimensions of the cranium in D. Terrdli 

 have not yet been fully made out, though several heads have been found, 

 and one of these is figured, credited to D. HertBeri, in Vol. I., Plate 33. 

 This shows only the posterior half. The anterior portion seems to have 

 consisted, originally, of several bones united by cartilage, for they are 

 always found dismembered and displaced. This is also the case with the 

 crania of the congeners of Dmichthys — Asterol&pis and Heterostius — of 

 the European Devonian. There was, however, a bony shell over the con- 

 necting cartilages, and, sooner or later, the head will, doubtless, be found 

 so complete that its form and the homologies of its component plates can 

 be fully made out. This I infer from the fact that a head of Dinichtliys 

 Hertzeri was found by Mr. Hertzer at Delaware, forming the nucleus of 

 a large concretion, and retaining nearly the natural position of all its 

 parts. The length of the head cannot be accurately determined, but it was, 

 probably, about three feet. Its width, at the broadest part, was, perhaps, 

 two feet. The largest cranium of D. TerrelU, found at Sheffield, 

 measures thirty inches, from angle to angle, across the occiput. 



The surface of the cranium was, as we know, gently arched, and, in its 

 general aspect, smooth. The external surface everywhere shows a fine, 

 almost microscopic figure, or grain, but nothing of the tuberculation com- 

 mon to all other known Placoderms. It is also marked, like Goccosteus, 

 by a series of linear furrows which form a kind of Arabesque pattern. 

 Whether the bones of the head of D. Hertzeri were similarly ornamented 

 we have not yet learned, as the plates of the head, which has been 

 referred to, and the only one found, are so much exfoliated that they show 

 no markings. 



The bones composing the cranium of D. Terrelli have rarely been 

 found disarticulated. We are therefore unable to compare them in de- 

 tail with those of J). Hertzeri, or those of other Ganoids. It is evident 

 that in the living fish they were firmly soldered together, and formed a 

 brain-case impenetrable to even the formidable dentition with which it 

 was associated. 



The occiput was symmetrically arched, the center, or keystone of the 

 arch being formed by the Supra-occipital. This bone is triangular in 

 outline, with a prominent point projecting from the middle of its longest 

 and posterior side. In its central part it is sometimes three inches in 

 thickness ; below it is excavated on either side for articulation with the 

 " ossa artioularia capitis " (Epiotics ?), and behind it slopes downward 

 and shows a broad, deep, and partially double pit. In D. Hertzeri, the 

 posterior margin of this bone is more nearly vertical, and bears at its cen- 

 tral point a pyramidal projection, as does the corresponding bone in He- 

 terostius ; anterior to this, and on the under surface of the thickest part, 



