56 COLORADO FORMATION AND ITS INVERTEBRATE FAUNA, [bull.ioo 



interruption, the young shell is usually found to be of an ovate form, 

 and attached by the whole under surface of the lower valve, the beak 

 of which is pointed, provided with a small triangular area, and usually 

 turned a little to the left. In this form they continue to grow to 

 lengths varying from 0.25 to 1 inch, when the margins are abruptly 

 deflected upward at right angles to the flat attached base, and pro- 

 duced in this direction often for as much as an inch or more; the great- 

 est extension being on the lateral margins and at the extremity oppo- 

 site the beaks. When seen at this stage of their growth, separated 

 from the body to which they were originally attached, and lying partly 

 embedded in the matrix, with the beak side down, they look like short 

 cylindrical tubes, with one end abruptly truncated and closed by the 

 flat surface of attachment; so that what was originally the whole 

 under surface of the valve now appears like the truncated umbo. 



" The other valve is quite flat, or sometimes a little concave, and 

 always retains the form possessed by the attached valve at the time its 

 margins became deflected upward, after which it seems to have in- 

 creased very little in size. Its umbo is usually a little less pointed 

 than that of the other valve, and provided with a shorter area, on each 

 side of which its margins are sometimes slightly crenulated. 



" The muscular impressions of both valves are obscure, and the sur- 

 face is nearly smooth, or only marked by line, indistinct lines of growth. 



u Locality and position, — At numerous places along the Missouri 

 between the Big Sioux and the Great Bend; also on the Little Blue 

 river, near the Kansas and Nebraska line, and near the Black hills, on 

 Cheyenne river, as well as on the North Platte; in the Niobrara group, 

 or formation No. 3, where it is usually found attached to fragments of 

 a large Inoceramus. It likewise occurs at several localities in New 

 Mexico and Colorado, probably in the same position." 



Ostrea solenisous Meek. 

 PL ii, Fig. 1; PL in, Figs. 1 and 2. 



Ostrea soleniscus Meek, 1871, Proc. Am. Philos. Soc, vol. xi, p. 430; 1873, Ann. Rept. 



U. S. Geol. Sur. Terr, for 1872, p. 487; White, 1880, idem for 1878, p. 9, PL 11, 



Figs. 2a, b; 1884, 4th Ann. Rept. U. S. Geol. Sur., p. 300, PI. 42, Fig. 1. 

 Ostrea cortex White, 1876, U. S. Geog. and Geol. Sur. West 100th Meridian, vol. iv, 



p. 170, PI. 15, Fig. 2a-c. 

 Not Ostrea cortex Conrad, 1857, U. S. and Mex. Bound. Sur., vol. i, p. 157, PI. 11, Fig. 



ia-d. 



u Shell attaining a large size, becoming rather thick in adult exam- 

 ples, generally straight, greatly elongated, and comparatively very 

 narrow, with parallel lateral margins. Lower valve with moderate 

 internal concavity, and having the appearance of a little gutter or 

 elongated trough; beak usually nearly straight, rather obtusely pointed, 

 and more or less distorted by the scar of attachment ; ligament area of 

 moderate size, strongly striated transversely, and provided with a large, 



