OARBONIFEROCrS SPECIES. 95 



it agrees in its extreme thinness as well as in general appearance. It, how- 

 ever, attains a larger size, and has proportionally larger costse ; while its 

 surface does not show, under a magnifier, the fine, regular, and distinct 

 striae of growth seen on that species. 



Among foreign species, it seems to be most nearly represented by Avi- 

 culopeden papyraceus, Sowerby (sp.); which, however, is a much larger, 

 more oblique shell, with very differently formed ears, as illustrated by Sow- 

 erby. In the form of its posterior ear, as well as in some other characters, 

 it agrees more nearly with a shell from the Coal-Measures of Belgium, fig- 

 ured by Professor De Koninck (An. Fos. Belg., plate v, figs. 6 a, h), and 

 by him referred to A. papyraceus, Sowerby ; though it differs materially in 

 other respects. If the figures published by Sowerby and De Koninck, of 

 the forms illustrated by them under the name Avicula papyracea, are exactly 

 correct, I should think they represent two distinct species. At least, none 

 of our analogous species in this country present, among hundreds of individ- 

 uals, such marked differences as are seen between their published figures. 



Locality and position. — From a black bituminous shale at Hamilton, 

 White Pine Mountains, Nevada ; of Devonian or Carboniferous age. 



AvicujLOPECTEN Qtahensis, Meek. 



Plate 9, figs. 7,7 a, 7 b (and 7 c, d?). 



Peeten Utahensis, Meek (1860), Proceed. Acad. Fat. Sci. Philad,, XII, 310. 

 Aviculopecten Dtahensis, Meek (1876), in Col. Simpson's Report Expl, across the Great 

 Basin of Utah, 354, pi. i, figs. 9 a, 9 b. 



Shell of about medium size, suborbicular, compressed, thin, not oblique; 

 hinge-line straight, equaling about one-half to two-thirds the antero-poste- 

 rior diameter of the valves ; pallial margin regularly rounded. Left valve 

 compressed, or moderately convex ; ears rather small, flattened so as to be 

 more or less distinct from the slight convexity of the umbo, each separated 

 from the margin by a shallow, obtusely angular notch, of which the one 

 under the anterior ear is rather more distinctly defined ; anterior ear nearly 

 rectangular at the extremity, with a slightly convex anterior margin; 

 posterior ear generally more obliquely truncated ; beak small, rather com- 

 pressed, not projecting above the hinge, and placed at or slightly in advance 

 of the middle of the cardinal margin, with its lateral slopes diverging at an 



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