CAEBONIFEEOUS PERIOD. . 141 



Genus RETZIA King, 1850. 

 Reztia Mormonii Marcou. 



Plate X, fig. 7 a, b, auA c. 



Terebratula Mormonii Marcou, 1858,* Geology of North America, 51. 

 Betzia punctilifera Shumard, 1858, Trans. St. Louis Acad. Sci., i, 220. 

 lietzia Mormonii Meek and Hay den, 1859, Proc. Acad. Nat. Sci. Pliila., 27. 

 Reizia suhglohosa McCbesney, 18G0, Descr. New Pal. Fossils, 45. 

 Retzia Mormonii Geinitz, 1SG6, Carbonformat. und Dyas iu Nebraska, 39. 

 Retzia punctilifera Meek, 1872, U. S. Geol. Surv. Nebraska, 189. 



Shell small, ovate in outline, both valves more or less gibbous ; hinge- 

 line short ; ears very minute, and observable only in well-preserved exam- 

 ples. Ventral valve a little more capacious than the dorsal ; posterior 

 portion narrowed to the umbo, which is prominent and considerably arched ; 

 beak small, truncated by a foramen of moderate size ; area small but well 

 defined. Dorsal valve almost as prominently convex as the ventral ; umbo 

 prominent ; beak incurved and extending a trifle over the cardinal border. 



Surface of each valve marked by from fourteen to seventeen simple, 

 nan'ow, radiating costse, having interspaces of similar width ; costse sharply 

 elevated, their backs, as well as the bottoms of the interspaces, somewhat 

 flattened ; mesial fold and sinus wanting or obsolete. 



This shell is a characteristic and not imcommon one in the Carbonif- 

 erous strata of Illinois, Missouri, Iowa, Nebraska, and Kansas, but it is 

 repi'esented by only a few examples in the collections. 



Position and localit'ij. — Strata of the Carboniferous period ; near Santa 

 Fd, New Mexico, and top of Grass Mountain, Ely range, thirty -five miles 

 north of Pioche, Nevada. Professor Marcou obtained his type-specimens 

 from Utah. 



Genus SPIEIGBRA d'Orbigny, 1847. 



Spirigera subtilita Hall. 

 Plate X, fig. 6 a, h, and c. 



Terebratula subtilita Hall, 1852, Stausbury's Exped. Great Salt Lake, 409. 

 Terebratula f subtilita Davidson, 1857, Monog. British Carb. Brach., 18. 

 Spirigera subtilita Meek and Haydeu, 1859, Proc. Acad. Nat. Sci. Phila., 20. 

 * See foot-note coucerniug date of iiublication ou page 125. 



