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



of the valves straight and sloping from the beaks to the posterior end, 

 capped or connected by a slender styliform, plain accessory plate; pos- 

 terior extremity small, truncated, or narrowly rounded; basal margins 

 nearly straight, connected by a ventral accessory plate similar to the 

 dorsal one, except that it is shorter, broadest behind, but coming to a 

 slender point in front about midlength of the shell, longitudinally divided 

 by a linear groove; front regularly rounded, both vertically and later- 

 ally; anterior gape consisting of a narrow, vertical slit, which occupies 

 the middle of a somewhat prominent projection at the antero-basal por- 

 tion of the shell, which projection has the shape of a Norman shield, as 

 seen by front view when both valves are in their natural position, and 

 which seems to have been occupied by a much wider gape in the younger 

 than in the adult condition of the shell; both umbonal grooves dis- 

 tinct, both upon the outer surface and upon that of the stony cast; 

 anterior grooves broader and deeper than the other, but both are 

 slender; besides the two umbonal grooves there is another somewhat 

 broader groove or furrow, extending with a broad, downward curve 

 from the posterior side of the beak to the posterior end of the shell. 

 This groove, like the others, is distinctly traceable upon the outer sur- 

 face, but is more distinctly seen upon the stony cast. 



"A broad, subcircular, cake-like umbonal accessory valve covers the 

 beaks and the space between them, the valve being divided by a suture 

 into two nearly semicircular pieces so neatly that it is hardly percepti- 

 ble until the valves are slightly displaced. The margins of the principal 

 valves between the beaks and the Norman shield-shaped projection are 

 narrowly but abruptly everted, which, with the beaks above and the 

 borders of the projection below, bound a distinctly hollowed space on 

 each side and below each beak. Besides the grooves before mentioned, 

 the surface is marked by fine concentric, distinctly raised lines on each 

 side of the shell, but they are less distinct upon the snrface of the 

 Norman shield-shaped projection than elsewhere. Between the pos- 

 terior grooves or furrow before mentioned as ending at the posterior 

 margin of the shell and the dorsal margin, the surface is occupied by 

 strong, irregular scales and laminae that were successively left as the 

 shell increased in size. 



" Length, 13 mm ; greatest height, 7 mm ; breadth at front, 6™." 



For extended comments on the generic relationship of this species, 

 see Dr. White's remarks in the second work above referred to. 



Comparison of this species with Parapholas californica, the type of 

 the genus, leaves little room for doubt that they are congeneric, but it 

 seems to me that it is still an open question whether Parapholas is 

 really distinct from Martesia. 



Locality and position. — Upper Kanab valley, Utah, and near the base 

 of the Coalville section. Specimens from the latter place are men- 

 tioned by Meek 1 as u Martesia (undetermined sp.)." 



»Aan. Kept. U. S. Geol. Sur. Terr, for 1872, p. 476. 



