118 COLORADO FORMATION AND ITS INVERTEBRATE FAUNA, [bull. 106. 



Locality and position. — In the Pugnellus sandstone on Williams 

 creek and in Poison canyon, Huerfano county, Colorado. At Coal- 

 ville, Utah, casts that are believed to belong to the same species were 

 found in the sandstone of the " first ridge," and also at a much higher 

 horizon in the " third ridge" of the same section. 



Genus LIOP1STHA Meek. 



LlOPISTHA (PSILOMYA) MEEKI White. 



PL xxvi, Figs. 5-7. 



Hopisiha (Psilomya) Meehi White, 1874, Expl. and Sur. West 100th Merid., Prelim. 

 Rept. Invert. Fossils, p. 26; 1876, U. S. Geog. and Geol. Sur. West 100th Meridian, 

 vol. iv, p. 186, PL 18, Fig. Ua-d. 



Eevised description : 



" Shell short, much inflated; umbones large, elevated; beaks small, 

 strongly curved inward and downward, and very slightly turned for- 

 ward ; posterior portion moderately produced, somewhat compressed 

 laterally; free margins forming a regular but unequally convex curve, 

 the greatest convexity of which is in front and the least along the base; 

 upper portion of the posterior border obliquely truncated, so that the 

 greatest posterior extension of the shell is a little below the hinge ex- 

 tremity. 



" Surface having a smooth aspect, but it is marked by fine concentric 

 lines of growth. Under a lens, very fine, obscure, radiating striae are 

 seen upon the surface of a little more than the anterior half of the 

 shell; and upon the remainder of the surface, except a small space ad- 

 joining the posterior cardinal border, there are small, somewhat distant, 

 radiating striae, easily seen by the unassisted eye. Upon these striae, 

 both the distinct and the obscure, the lens shows numerous minute 

 punctures, placed at irregular intervals, which are the bases of minute, 

 short, blunt spines, or which mark the places from which the spines 

 have been removed. 



"Length, 25 m,n ; height, from base to umbo, 20 mm ; greatest thickness, 

 both valves together, 16 m,n . 



" This shell seems to be more nearly related to L. globulosa (—Poromya 

 globulosa Forbes) than to any other described species. Compared with 

 that species, as figured and described by Stoliczka, 1 ours differs in be- 

 ing less globular, in having the umbones more elevated, and in the more 

 distinct radiating striae upon the posterior half of each valve." 



Locality and position. — The types come from southeast of Paria, Utah. 

 In the Upper Kanab valley, which is not far from the typical locality, 

 it occurs about 350 feet above the base of the Cretaceous section, asso- 

 ciated with characteristic species of the Colorado formation. 



1 Cretaceous Fauna of Southern India, vol. ui, p. 47, PI. in, Fig. 8, and PI. xvi, Fig. 16. 



