30 PALEONTOLOGY OF OHIO. 



are smaller, being respectively eighteen and twenty inches long, but 

 both these are much worn, as though belonging to mature individ- 

 uals. ■ In all these the form is essentially the same, the anterior ex- 

 tremity is turned up, and terminates in a strong, acute, tooth-like 

 projection. This is much worn, and was maintained in a sharp state 

 by friction with the " premaxillary," into the concavity of which it 

 enters. Behind this great tooth is a triangular, flattened projection, 

 formed by a ridge on the inside of the mandible. Back of this, the 

 upper margin of the mandible, for about six inches, is sharp, and is com- 

 posed of dense, enamel-like tissue. At the posterior end of this sharp 

 edge tubercles may frequently be discovered, which seem to be the rudi- 

 mentary representatives of the teeth that surmount the margin of the 

 mandible in D. Hertzeri.^ Usually the edge of the mandible is worn 

 and sharp, from contact with the maxillary. The outline of this portion 

 of the mandible is not all shown in Fig. 6 of Chart Y., as both the 

 triangular denticle and the cutting edge are broken away. 



From the posterior end of the Icnife-edge of the mandible, a distinct 

 shoulder runs in a curved line downward and forward to the beginning of 

 the anterior curve. Above and before this shoulder the mandible is thick and 

 massive, and was, evidently, never covered. The surface is nearly smooth, 

 but shows everywhere the fine, granular reticulation which characterizes all 

 the external surfaces of the bones of Dinichthys. The great terminal 

 " tooth " is smooth, and wants the line of tubercles found on this part of 

 the mandible of D. Ilertzeri. The posterior portion of the mandible is 

 flattened and smooth ; in outline it is more spatulate than the correspond- 

 ing portion of this bone in the other species, and is more turned up. Ail 

 this part, as far forward as the shonlder referred to above, was, evidently, 

 once covered with integument, or spliced on to the cartilage which formed 

 the articular extremity. The more prominent characters mentioned in the 

 above description will be seen in the full-sized figure of a mandible of D. 

 Terrelli, represented on Chart V. (Fig. 6). 



Body Plates. — The dorsal shield of D. Terrelli is so well shown in 

 Fig. 5, Chart V., that no lengthy description is required of it. In the 

 mature individual it is about two feet in length and breadth, one side 

 being evenly rounded, the opposite one irregularly emarginate. Trans- 

 versely, it is strongly arched ; in its antero-posterior diameter, nearly 

 straight. The external surface is smooth or granulated. Below, it bears 

 along the central line an elevated, compressed ridge, which, at the curved 

 margin of the shield, rises four inches from the inner surface, and has a 



* A precisely similar crenulation is visible on the correspondiug portion of tlie lateral 

 dental plates (" maxillaries ") of Lepldosiren. 



