NOTES ON THE OHIO SHALES AND THEIR FAUNAS 29 



converge toward a point about one centimeter behind the 

 symphysis and near the top of the mandible. 



The mandible of this species differs from that of Stenosteus 

 glaber Dean in the anterior end curving strongly upward, in the 

 teeth decreasing in size posteriorly, and in the greater com- 

 parative width at the posterior end. 



Formation and locality : Fifty feet from the bottom of the 

 Ohio shales, one-eighth mile south of Monroeville, Ohio. 



Number 1496, Geological Museum of Oberlin College. 



Dinichthys subgracilis, sp. nov. (Plate II) 



Type an incomplete mandible. Portion anterior to the 

 main cusp missing, posterior third missing. Impression of the 

 posterior third preserved in the rock and reproduced in outline 

 in the figure. Length of mandible about 35 cm., length behind 

 cutting blade about 19 cm., width at posterior end of cutting 

 blade 62 mm., greatest width posterior to cutting blade 65 mm., 

 thickness immediately behind cutting blade 13 mm., gradually 

 thinning to 9 mm. twelve centimeters behind the cutting blade. 

 The cutting blade shows one tubercle just behind the main 

 cusp. The posterior end of the mandible curves strongly up- 

 ward for the last ten centimeters but maintains a nearly uni- 

 form width. 



Comparing this mandible with one of Dinichthys inter- 

 medius of the same length the latter is found to be nearly twice 

 as wide in the widest part behind the cutting blade, the cutting 

 blade is one-half wider, and the entire mandible is much thicker 

 and stronger. As the mandible has one minute tubercle on 

 the cutting blade it may be ancestral to the stronger non- 

 tuberculate forms of the upper part of the Ohio shales. 



Formation and locality: Top of Olentangy shale, two 

 miles west of Huron, Ohio. 



