672 



GEOLOGY OF OHIO. 



scar which in casts appears as a sharply elevated point situated close to 

 the hinge line between the base of the beak and the upper part of the 

 anterior adductor scar. 



The total number of species now known to belong to this genus is 

 seven. With one exception, /. unionoides, Meek sp., which belongs to 

 the middle beds of the Cincinnati group, all are restricted to the upper 

 one hundred feet of the same formation, leaving over three hundred feet 

 of strata between the known first and second appearance of the genus in 

 which it is as yet unknown. Respecting the origin of the Ischyrodonta 

 type of structure, I am obliged to confess total ignorance. 



Ischyrodonta truncata, Ulrich. 



Ischyrodonta truncata, Ulrich, 1890, Amer. Geol., vol. vi, p. 174; S. A. Miller, 1892. 

 Appendix to N. A. Geol. and Pal , p. 700. 



Fig.l. Ischyrodonta truncata, Ulrich, Cincinnati group Oxford Ohio, a, outline view of a lei t 

 valve of this species, slightly restored. In some specimens the anterior margin slopes back- 

 ward more rapidly and in accordance with the inner line; b, internal view of same, showing 

 only the hinge plate, cardinal teeth, and anterior muscular impression, the cavity being filled 

 with adhering matrix ; c, d and e, three views of a very nearly perfect cast of the interior. Be- 

 tween the beaks of the central figure is shown a thin film of stone that had originally filled a 

 narrow interstice between the hinge plate and cardinal teeth. 



Shell of medium size, rather strongly convex, subquadrate to tri- 

 angular-ovate, highest posteriorly. Cardinal margin straight or very 

 slightly arcuate, more than three-fourths the length of the shell; anterior 

 end very short, narrow, rounding almost uniformly into the obliquely 

 convex basal margin, from which the outline turns rather sharply up 

 into the truncate-rounded posterior margin; post-cardinal region sub- 

 angular. Beaks small, umbonal ridge inconspicuous, no mesial sulcus. 

 Surface of the thick shell with a small number of sublamellose lines of 

 growth. 



