608 GEOLOGY OF OHIO. 



portion, the difference is obvious. The large Haversian canals of the 

 former are represented by the merest traces ( fig. 5). Successive deposits 

 of bone on the inner faces of the walls have so far lessened their diameter 

 that they are almost effaced while the solid tissue filling their place is 

 crowded with lacunae and traversed in ever}' direction and in every 

 spot with canalicules forming a perfect and minute net-work over the 

 field of the microscope — a feature which could not be well shown in the 

 same figure without overcrowding it, but which is represented in a small 

 degree in fig. 5. 



This dense bony tissue forms upwardly the inner edge of the man- 

 dible, and as the shear-tooth of the upper jaw closed down outside of it 

 the constant use and wear ground down the outer soft tissue and kept 

 a permanent, sharp edge upon the inner bone thus exposed, as shown in 

 figure 1. 



It is not easy to see with such a structure of the dental apparatus 

 how an) T renewal of the tissue was possible, and probably the life of the 

 creature was limited to the duration of its mandibular edge. That very 

 extensive wear actually occurred is evident from the appearance of 

 specimens which have been found and which show no indication of 

 repair. This is contrary to the usual structure of fishes where provi- 

 sion is generally found for the renewal of the dentition almost ad infinitum. 



This dense tissue of the mandible in no respect resembles true tooth 

 structure. There is no trace of dentine or of osteo-dentine, still less of 

 enamel. It is true bone, but exceedingly dense and hard, and consequently 

 more resembling the cementum of the teeth of the higher vertebrates. 

 In this respect however it is in close accord with the tooth-structure of 

 most fishes which is traversed by similar canals and contains abundance 

 of lacunae and of inosculating canalicules. It is, in fact, a kind of trans 

 itional materal between typical bone and typical tooth. 



Titan ichth vs. 



When Dr. Newberry published his "Fossil Fishes of North America," 

 he mentioned two species of this genus. These were all that were then 

 known from Ohio, and the genus has not yet been reported elsewhere.* 

 Of these two species Titanichthys Clarki was the larger and heavier form 

 T. Agassizi the lighter and slenderer. The type of the former is in the 

 Museum of Columbia College, in New York, and that of the latter in the 

 Museum of Comparative Zoology at Cambridge. 



In the present state of our knowledge it is not possible to charac- 

 terize the whole fish. Only separated and scattered parts have thus far 

 been found, except in a few cases. Of these parts the most frequent and 



* Titanichthys Pharao, of Dames, from the Cretaceous of Egypt of course drops 

 as a synonym, being published only in 1887. This fossil has no connection -with 

 those here mentioned. 



