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



small triangular anterior alation, is oblong oval. Upper valve nearly 

 flat or slightly concave, excepting on the posterior margin, where there 

 is a broad downward flexure corresponding to the emargination and 

 the convexity of the lower valve. Surface of both valves marked only 

 by lines of growth and a few concentric lauiinse. 



The largest examples measure 45 mtn from beak to base, and about the 

 same in the greatest length at right angles to that line. 



The numerous examples from several localities show less variation 

 in form than is usual in species of Ostrea. Its outline suggests 0. mal- 

 leiformis Gabb from the Ohico group of California, but the differences 

 seem to be sufficiently great and constant to warrant their separation. 



Locality and position. — In the Pugnellus sandstone near Malachite 

 post-office, Poison canyon, and other localities in Huerfano park, Colo- 

 rado, and at about the same horizon 20 miles above Pueblo. 



Ostrea ltjgubris Conrad. 

 PI. iv, Figs. 1-10. 



Ostrea lugiibris Conrad, 1857, U. S. and Mex. Boundary Rept., vol. I, p. 156, PI. 10, 

 Figs. 5a, b; Meek, 1876, Macomb's Expl. Exped. from Santa F6 to Junct. of Grand 

 and Green rivers, p. 123, PI. 1, Figs, la-d; White, 1884, 4th Ann. Rept. U. S. Geol. 

 Sur., p. 297, PL 51, Fig. 3. 



Ostrea bellaplicata Sliuniard, 1860, Trans. St. Louis Acad. Sci., vol. l, p. 608; White, 

 1879, Ann. Rept, U. S. Geol. Sur. Terr, for 1877, p. 276, PI. 4, Figs. 3a, b, and PI. 8, 

 Figs. 2a, b; White, 1884, 4th Ann. Rept. U. S. Geol. Sur., p. 292, PI. 78, Figs. 1, 2, 3. 



Ostrea (Alectryonia) blackii White, 1880, Proc. U. S. National Museum, vol. n, p. 293, 

 PI. 4, Figs. 1, 2 ; Ann. Rept. U. S. Geol. Sur. Terr, for 1878, p. 11, PI. 14, Figs, la, b, 

 PI. 17, Fig. 4; 4«h Ann. Rept. U. S. Geol. Sur., p. 292, PI. 45, Fig. 1, and PI. 46, Fig. 2. 



Shell varying in size from small to medium; outline usually broad, 

 subovate, but in small specimens often nearly circular and in larger 

 ones occasionally subtri angular; lower valve moderately convex, the 

 greatest convexity being along the median line, which is often sub- 

 angular in large shells; beak usually small and inconspicuous, but some- 

 times more prominent and curved laterally, often obscured by the scar 

 of attachment, which is usually present and is proportionately very large 

 in the dwarf variety represented by Conrad's type. Surface marked by 

 from twelve to eighteen strong plications, that radiate from the beak or 

 the scar of attachment, and by strong concentric lines, and sometimes by 

 imbrications of growth. The plications are usually simple, but occa- 

 sionally they bifurcate. Upper valve nearly flat or sometimes slightly 

 concave, with an outline similar to that of the other valve, excepting 

 that it is somewhat more narrow along the hinge line. Its surface is 

 smooth for some distance around the beak and the plications toward 

 the margin are not as strongly developed as on the lower valve, which 

 it resembles in other respects. The muscular scars are reniform and 

 subcentral ; ligamental area varying greatly in size and form, but never 

 very large. 



