FOSSIL FISHES. 607 



obliged to consider it new. It is represented on Plate XLJ fig. I. A 

 glance will bs 'sufficient to assure any one familiar with the genus that it 

 can belong to none of the four heavy forms — D. Hertzeri, D. Terrelli, 

 D. curtus and D. intermedins. From D. minor and D. Gouldi it differs 

 in its greater slenderness and from the former also in its lack of the 

 second denticle. It more resembles D. corrugatus but surpasses even 

 this in lightness. With D. tuberculatus we cannot compare it as the jaw 

 of that species is unknown. Moreover, it was found in Pennsylvania. 

 The upturned end of the mandible is almost at right angles with the 

 shaft which carries on the upper edge a thin wing of bone beveled off 

 to a feather blade and continued up the hinder face of the front tooth. 

 The tipper edge of this blade is not even, being low in front as shown 

 in the figure. But there is no sign of the trenchant cutter which char-' 

 acterizes several species of this genus and indeed with so thin and slender 

 a mandible such a blade would have been powerless, for this jaw can 

 hardly have been employed as a weapon of offense by its wearer. The 

 narrowness of the spatulate expansion at the back would also indicate 

 that the motor-muscles were less massive than in the more formidable 

 species. So far, as I am aware this is the only part of this Dinichthys 

 that has yet come to light and it was found in Cuyahoga county , O., and 

 I have named it D. gracilis from its. slender build. 



Structure of the teeth of Dinichthys. 



The shear-blade of the mandible of Dinichthys shows an interesting 

 microscopical structure well adapted to the work which it performed. 

 As many artificial cutting-tools it is composed of a dense material forming 

 the edge backed by a less dense and brittle substance , to sustain the 

 pressure. 



A polished section of the mandible is represented on fig. 1. PI. 43 . 

 The inner part is of a deep brown color, almost black, solid, and to the 

 eye homogeneous. The outer part is dense in texture and lighter in tint. 

 When cut or ground down thin for microscopic examination the two 

 parts as shown in the small figures present very different appearances. 

 The open and spongy tissue of the jaw (figs. 2, 3) is traversed by a great 

 number of very wide Haversian canals with thick walls in which are 

 scattered the lacunae or osteoblasts in moderate abundance with their 

 accompanying canalicules. The Haversian canals are so large and 

 numerous that they occupy most of the space, the solid portion of the 

 bone consisting merely of their walls and these do not exceed in thick- 

 ness the diameter of the canals. These latter are filled with pyrites 

 which is perfectly opaque and shows in clear distinction from the bone 

 which is stained dull red with infiltered material. 



On contrasting this structure which characterizes all the bony plates 

 of Dinichthys as well as the jaw with that of the inner and solid dense 



