GIRTY, NEW FOSSILS FROM THAYNES LIMESTONE 341 



defined, a small portion of each near the hinge slightly upturned. This flat- 

 tened area, which may possibly be regarded as the entire wing, is larger and 

 more sharply defined on the posterior side. 



Surface nearly smooth, marked by fine, somewhat irregular and unequal 

 concentric striae and by obsolescent radial costse. Of the latter, 9 or 10 are 

 stronger than the rest, but between these and at the sides are other very fine 

 obscure raised lines, S or more between 2 of the stronger ones. The whole 

 sculpture is rather irregular and so fine and obscure as to be hardly visible 

 without a lens. 



The type specimen, from the absence of a byssal sinus, is evidently «. 

 left valve, unless the two sides of the shell are similar in configuration, 

 in which case the genus would be neither Pecien nor Aviculipecten. 



In its specific relations, this form is doubtless most closely allied to 

 -i. occidaneus, from which it is distinguished by some differences in 

 shape, such as the less well-defined wings and by the nearly obsolete 

 sculpture 



Aviculipecten w^asatchensis sp. nov. 



Shell small, subovate, distinctly longer than wide. Hinge line straight, 

 slightly shorter than the greatest width below. Ventral margin regularly 

 rounded. Sides nearly straight, gently contracting toward the hinge near 

 which they curve slightly outward. 



Convexity moderately high, rather strong at the umbo. A small triangular 

 area either side of the beak is slightly flattened into an undefined wing. The 

 sides below these areas are deflected from the body of the shell, forming a 

 larger feature, which may also be regarded as the wing instead of the other. 

 Beak nearly central. Axis about perpendicular to the hinge. 



Surface marked by fine, regular, closely arranged concentric striae and by 

 radial costfe of several sizes. Between each pair of larger ribs are one or two 

 distinctly smaller, while a third series, one or two at a time, occurs between 

 the second ones. This arrangement is subject to more or less irregularity, 

 and toward the sides the costae tend to be more distinctly of one size, but the 

 general effect is that of regularity. The costte are not sharply or strongly 

 elevated and do not extend onto the sides. 



The absence of any sinus for the byssus indicates that the specimen 

 described is a left valve. The right valve is unknown. 



This species differs from A. boiitweUi in having the costje stronger 

 and more conspicuously alternating and in having sharper and more 

 regular concentric striae. 



Aviculipecten thaynesianus sp. nov. 



Shell small, subovate, longer than wide. Hinge line straight and shorter 

 than the extreme width below. Ventral margin regularly rounded. Sides 

 nearly strniglit. curving gradually below into the ventral outline and contract- 

 ing slightly toward the hinge. 



