614 GEOLOGY OF OHIO. 



hard mouth parts either for capturing or for mastication. The causes 

 which produced the retrogrde development in the jaws of some modern 

 Sirenians may have wrought with corresponding effect upon the jaws of 

 Titanichthys. 



The details of the structure of the anterior part of the mandible 

 are exhibited in the series of cross sections given in figure 2 of the plate. 

 The posterior portion of the mandible was not preserved; but if it was 

 as long proportionally as in the other species the entire length may have 

 reached thirty inches. This description was contributed by Prof. A. A. 

 Wright. 



Gorgonichthys Clarki, CI. 



Formidable as was the dentition of both the species of Dinichthys, 

 the researches of Dr. Clark in the Cleveland shale have lately been 

 rewarded by the discovery of an armored monster whose offensive weap- 

 ons surpassed even those carried by these terrible fishes. In 1891 he 

 found a pair of mandibles presenting characters not previously recognized 

 and with them a mass of bony plates evidently belonging to the same 

 creature. Little of the material has } r et been worked out, but following 

 the principle here adopted of defining the species as far as possible by 

 the lower jaws, they were first extracted from the matrix. It then became 

 obvious that in several respects the}' differed from any previously known. 

 The shearing tooth and edge of the lower mandible were lacking. In 

 place of the latter was a blunt process rising at some distance behind the 

 great front tooth and set with rounded tubercles on its fore and hind 

 slopes. Opposed to this in the upper jaw was an enormous doubly 

 pointed tooth so set as to play on the top of this process which was 

 received between its two points. This character alone is sufficient for 

 the identification of the genus if the mandibles are obtainable. No doubt 

 when the rest of the plates have been extracted from the matrix, or other 

 specimens fouud, many more characters will be determinable. 



The great front tooth so closely resembles that of Dinichthys that no 

 special description is requisite. Its antagonist in the upper jaw closed 

 down behind it as usual in Dinichthys but only its tip was present in this 

 specimen, the rest having been broken off at or before its discovery. 

 This tip la}' in the groove which it had worn in the back part of the 

 lower tooth as shown in the figures, but for the sake of clearness it has 

 been represented lying entirely clear. See PiateXLI. 



In size the mandible of Gorgonichthys about equals that of Dinich- 

 thys, being about 25 inches in length. But it is considerably heavier and 

 better adapted to meet the greater weight of the opposing tooth in the 

 upper jaw. This tooth measures nine inches from front to backb}' seven 

 in a vertical direction. Like the rest it consists of the peculiarly hard, 

 black, dense, bony tissue of which the shear-tooth and blade of Dinichthys, 



