LOWER SILURIAN LAMELLIBRANCHS. 677 



fact that it has the shell structure, cardinal teeth and small anterior 

 pedal muscles of a true Ischyrodonta. 



Formation and Locality: Upper beds of the Cincinnati group, Rich- 

 mond, Indiana. 



Ischyrodonta unionoides, Meek. 



Plate 54. Figs. 1-3. 



Anodontopsis ? unionoides, Meek, 1871, Amer. Jour. Sci. aud Arts, Vol. II, p. 299. 

 Anodontopsis (Modiolopsis?) unionoides, Meek, 1873, Pal. Ohio, Vol. I, p. 141. 

 Modiolopsis unionoides, S. A. Miller, 1890, N. Amer. Geol. and Pal., p. 491. 



Shell of medium size, subovate, a little the highest posteriorly, com- 

 pressed 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, projecting 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 dis- 

 tinguishable. Surface showing only a few distant subimbricating 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 ex- 

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

 of the length of the shell. Anterior adductor and pedal muscle attach- 

 ments 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. 



Meek's type of this species has almost beyond question, been dis- 

 torted by pressure so that its height is now less than it should be. In 

 one of my specimens the height has been reduced to an even greater de- 

 gree. The second specimen mentioned by Meek {loc. cit.), which he re- 

 fers to the species with doubt because it is proportionally higher, seems 

 to agree exactly with those now illustrated on plate 54, and which I re- 

 gard as representing the normal form of the shell. 



Having the hinge and muscular impressions characterizing Ischyro- 

 donta, and the shell structure prevailing among the Cyrtodontidcs ; the 

 species cannot possibly belong to Modiolopsis nor to any other genus of 

 that family. As to placing the shell with McCoy's Anodontopsis, it is 

 out of the question if McCoy has defined his genus correctly. 



The rather regularly ovate form of I unionoides suggests relation- 

 ship to /. oralis. It is however a larger shell, relatively higher, es- 



