- TEENTON TERIOD. 73 



strong, supported by a thin lamina in young shells, and by a still greater 

 thickening of shell-substance beneath them in older ones. 



Ventral valve gently arching from beak to front, and also from side to 

 side ; greatest convexity near the beak, which is abruptly pointed and pro- 

 jects backward beyond the hinge-line ; area wider than that of the dorsal 

 valve, and like that area it diminishes to a point at each extremity of the 

 hinge ; foramen triangular, extending to the apex of the beak. 



Surface of both valves marked by strong, elevated plications, which 

 are slightly flattened, or have a narrow linear depression along the back, 

 each interspace also having a corresponding slender, slightly-raised line 

 along its middle. So far as observed, all the plications extend without 

 interruption from the umbonal region of each valve to the margins. The 

 plications are shown distinctly upon the inner surface of the valves, especi- 

 ally at the margins, where also slight linear depressions are seen that mark 

 the places of the raised lines between the phcations on the outer side. 

 Very fine concentric striae are visible under a lens upon the outer surtace, 

 which, in consequence of erosion probably, are usually more distinct between 

 the plications than upon them. 



This shell is referred with doubt to 0. plicatella Hall. It agrees with 

 that shell in general characters, and yet it presents differences that are at 

 least as great as those which separate 0. fissicosta Hall from 0. plicatella. 

 It is also much like 0. tricenaria Conrad in some of its features, but seems 

 to differ from that shell as much as from 0. plicatella. The differences from 

 the latter are the continuity of all (?) the plications from the umbonal region 

 to the margin without bifurcation or implantation, the slight flattening or 

 depression of the back of each plication, and the presence of the slightly- 

 raised lines at the bottom of the interspaces. Since the species is known 

 to be a variable one, I am disposed to regard these differences as only 

 varietal until farther comparisons can be made. 



Length of one of the largest specimens in the collection, fifteen milli- 

 meters ; breadth, eighteen millimeters ; distance from center to center of 

 the plications at the front margin, about one and a half millimeters. 



Position and locality. — Strata of Lower Silurian age, probably of the 

 Trenton period ; Fossil Butte, near Hico, Nevada. 



