MARYLAND GEOLOGICAL SURVEY 281 



Occurrence. MARTINSBURG SHALE (Eden division). Eickard Moun- 

 tain, Washington County, Maryland. Eden shale and Maysville group of 

 New York and Ohio. 



Collection. U. S. National Museum. 



Family CYRTODONTIDAE 



Genus ISCHYRODONTA Ulrich 

 ISCHYRODONTA UNIONOIDES (Meek) 



Plate LVIII, Figs. 2, 3 



Anodontopsis ? unionoides Meek, 1871, Amer. Jour. Sci., vol. ii, p. 299. 

 Anodontopsis (Modiolopsis ?) unionoides Meek, 1873, Pal. Ohio, vol. i, 



p. 141, pi. xii, figs. 2a, b. 

 Ischyrodonta unionoides Ulrich, 1893, Geol. Surv. Ohio, vol. vii, p. 677, 



pi. liv, figs. 1-3. 

 Ischyrodonta curia Foerste, 1914, Bull. Sci. Lab. Denison Univ., vol. xvii, 



p. 298, pi. iii, fig. 14. 



Description. " Shell of medium size, subovate, a little the highest 

 posteriorly, compressed, convex, thickest slightly above and in advance of 

 the middle. Anterior margin regularly but rather narrowly rounded; 

 base forming a broad semielliptic curve; posterior margin broadly 

 rounded, very slightly oblique; dorsal outline more or less strongly 

 arcuate, passing gradually into the ends. Beaks small, compressed, pro- 

 jecting very little beyond the hinge margin, placed between one-fourth 

 and one-fifth of the length of -the valves behind the anterior extremity ; 

 umbonal ridge scarcely distinguishable. Surface showing only a few dis- 

 tinct subimbri eating marks of growth. 



" Hinge comparatively weak for the genus, with one oblique cardinal 

 tooth in the right valve and two (?) in the left. The ridge-like internal 

 ligament support leaves a linear depression within the dorsal edge extend- 

 ing posteriorly from the beak for a distance equaling about one-third of 

 the length of the shell. Anterior adductor and pedal muscle attachments 

 having the characters usual for the genus, except that they are, with 

 respect to the beaks, more anterior in position for the reason that the 

 anterior end is uncommonly long." Ulrich, 1893. 



