496 THE PALEONTOLOGY OP MINNESOTA. 



lOlionyehla ereota— rhomboldea. 



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 loeality.— Central part of the Trenton limestone at Minneapolis, Minnesota. 

 Mus. Beg. No. 5099. 



Cliontohia eeeota Hall. 



PLATE XXXV, FIGS. 17 and 18. 



Ambonyohia ereota 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.— Ttentoix limestone Beloit, Wisconsin, and Minneapolis, Minnesota. 



ClilONYCHIA EHOMBOIDEA TJlvich. 

 PLATE XXXV, FIGS. 19 and 20. 



Clionychia rhomboidea Ulbich, 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 teriiiinal, 

 small, pointed, projecting slightly above the hinge line, scarcely incurved. Umbonal 

 ridge strongly convex, extending toward the posterO-ventral extrefmity 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 



