40 
BULLETIN OF THE LABORATORIES 
Microdojh icf. subelliptica, Hall.) 
( Plate IV, Fig. 32. ) 
Both valves are preserved and are nearly alike. Shell very small, 
somewhat trapezoidal, obliquely truncate behind, height over four- 
fifths the length, gently and evenly convex ; dorsal margin, behind the 
beaks nearly straight, the gently concave cardinal margin forming 
nearly a right angle with it; lower margin a uniform curve. Posterior 
umbonal region evenly convex, but with evident ridge. The surface 
of the shell is marked by fine yet distinct concentric lines, there being 
about 21 upon a shell .20 high. Although the hinge has not been 
seen it seems reasonably sure that the generic reference is correct, 
making the first instance we remember of its occurrence in the coal- 
measures. 
Genus nuculana. 
J^'uculana heUistviata, Stevens. 
(Plate IV, Fig. 26. ) 
This very pretty shell is abundant. Our specimens are much 
smaller than the types. They resemble in form and markings the 
European Leda attenuata, yet probably are correctly identified with 
the West Virginia species. The species occurs also in Indiana. 
Genus Schizodus, King. 
Shell trigonal, rounded anteriorly, attenuated behind; rather thin, 
smooth, with an obscure oblique ridge; ligament external ; hinge teeth 
2.3, rather small; anterior adductor slightly impressed, removed from 
the hinge, with a pedal scar close to it ; pallial line simple. 
The reference of species to this genus upon the evidence of ex- 
ternal form alone, can only be provisional. There are over twenty 
species of fossil forms attributed to this genus. 
Six species from the coal-measures of this country are described, 
while there seem to be at least four forms among the specimens de- 
rived from the shales at Flint Ridge. It is frequently difficult to de- 
termine what variations are due to distortion, especially in thin-shelled 
specimens like these, which are usually more or less flattened. The 
