THE LOWER SILURIAN ROCKS. 651 



One is from the lower beds (Utica horizon) and of considerable interest 

 because of its oblique shape and consequent approximation toward the 

 upper Silurian and Devonian genus Mytilarca. The other is from the 

 upper beds of the formation and has an erect form like the- typical lower 

 Trenton species of the genus. 



Clionychia subundata, n. sp. 



Clionychia subundata, Ulrich. Left valve of the natural size. 



Shell less than the average size for the genus, moderately convex,, 

 oblique, the outline obscurely rhomboidal; anterior and basal margins 

 gently rounded, oblique, post-ventral extremity strongly rounded, pos- 

 terior margin but little convex; hinge line straight, about half as long as 

 the greatest oblique diameter of the shell; post-cardinal angle about 125°, 

 antero-cardinal angle between 65° and 70°. Beaks moderately promi- 

 nent, not strongly incurved. Surface marked with fine concentric line s 

 of growth and irregular concentric undulations, generally strongest on 

 the anterior slope. Interior unknown. 



This species is smaller and more oblique than any other now referred 

 to the genus. C. rhomboidea Ulrich, of the lower Trenton (Birdseye 

 limestone) in Minnesota, is the most like it so far as shape is ■ concerned,, 

 but the concentric undulations are believed to indicate closer affinities 

 with C. undata Emmons, sp. In the latter the surface undulations are 

 much broader, the hinge longer, and the shell much less oblique. 



Formation and locality: Lower beds (Utica horizon) of the Cincin- 

 nati group, at Covington, Kentucky; associated with Leptobolus lepis, 

 Hall. 



Clionychia excavata, n. sp. 



Plate 51, Figs. 4 and 5. 



Shell, as seen in a cast of the' interior, of medium size, erect, strongly 

 convex, subquadrangular, straight above, slightly sinuate anteriorly, and 

 rounded below and posteriorly; post-cardinal angle obtuse, perhaps 

 rounded; length of hinge line about two-thirds of the greatest width 

 (length) of the shell ; length and heighth respectively as six is to seven. 

 Beaks compressed, scarcely projecting above the hinge, separated by an 

 unusually wide interval; between and beneath them the greater part of 

 the upper half of the anterior side of the shell is deeply excavated. 



