496 THE PALEONTOLOGY OF MINNESOTA. 
[Clionychia erecta—rhomboidea. 
respects: The umbonal slope is less defined, the whole surface being more uni- 
formly convex, the beaks not so attenuate and more incurved, and the concentric 
growth lines not nearly so sharp, much more numerous and more equal. Casts of 
the interior are almost smooth, and the shell substance must have been very thin. 
The anterior side also is less concave, the shell smaller and the valves proportionally 
a little more convex. 
Formation and locality.—Central part of the Trenton limestone at Minneapolis, Minnesota. 
Mus, Reg. No. 5099. 
CiionycHiA ERECTA Hall. 
PLATE XXXV, FIGS. 17 and 18. 
Ambonychia erecta HALL, 1861. Rep. Sup’t. Geol. Sur. Wis., p 32. 
This species also is exceedingly like C. lamellosa, and for a time I was inclined 
to question the propriety of maintaining it. A more careful comparison, however, 
has revealed slight peculiarities that cause me now to view the separation with some 
favor. The valves of C. erecta are not so convex and more nearly square, the outer 
side being almost vertical and more produced below, the posterior side is straighter 
above and the postero-cardinal angle sharper. In all other respects the two forms 
are, so far as we can learn, identical. C. nitida is more oblique, its valves more 
convex and their surface markings finer. 
Formation and locality.—Trenton limestone Beloit, Wisconsin, and Minneapolis, Minnesota. 
CLIONYCHIA RHOMBOIDEA Ulrich. 
PLATE XXXV, FIGS. 19 and 20. 
Clionychia rhomboidea ULRICH, 1892. Amer. Geol., vol. x, p. 97. 
Shell, as seen in casts of the interior, of medium size, very oblique, rhomboidal 
in outline, the anterior and posterior and the dorsal and ventral margins subparallel. 
Dorsal edge nearly straight, likewise the posterior, the two lines meeting at an angle 
of about 120°. Postero-ventral margin sharply curved, the ventral side gently convex 
and rounding almost uniformly up to the base of the anterior side, from which point 
the outline continues to the beaks in very nearly a straight line. Beaks terminal, 
small, pointed, projecting slightly above the hinge line, scarcely incurved. Umbonal 
ridge strongly convex, extending toward the postero-ventral extremity in a slightly 
curved direction, so that the slopes on the anterior and ventral sides are more abrupt 
than on the opposite sides. Point of greatest convexity a little in front of and above 
the middle. 
Interior with hinge plate rather wide and strong, and the anterior edges of the 
valves, for a short distance beneath the beaks, much thickened inwardly, the decay 
