64 PAL.^.ONTOLOGY. 



of this shell. It h.as much the general aspect of 0. Michelini, but differs 

 rather decidedly fi-om any authentic variety of that species figured in 

 foreign works in its longitudinally oblong outline, straight and parallel 

 lateral margins, and greater convexity, especially that of its ventral valve, 

 which in 0. Michelini is most generally flat, or even a little concave, around 

 the anterior region. Its area is also proportionally smaller than in 0. Mich- 

 elini. In some respects, it agrees more nearly with a South American fonn 

 described by Professor Derby under the name 0. Penniana, already cited. 

 Still, it differs quite strongly in its oblong or quadrate form (produced by 

 the straightness and parallel outline of its lateral margins, which cause the 

 breadth of its valves to be almost exactly as great posteriorly as towai'd the 

 front ) ; while 0. Penniana is decidedly wider anteriorly than behind, almost 

 exactly as in the typical form of Michelini. If 0. Penniana of Derby is 

 really specifically distinct from 0. Michelini, which I am rather inclined to 

 doubt, then our shell might with more propriety be an'anged as a variety 

 of the same than of 0. Michelini, unless it may present some internal differ- 

 ences. If distinct from the above-mentioned forms, either specifically or as 

 a variety, it may be designated by the name Nevadensis. 



Locality and position. — From the Carboniferous limestones of White 

 Pine County, Nevada, twenty-five miles east of Hamilton, on the Egan 

 Road; also, at Railroad Canon, Diamond Mountains. Colonel Simpson 

 also brought it from the pass between Desert and Pleasant Valleys. 



PRODUCTID^. 



Genus PRODUCTUS, Sowerby. 



Peoductus Nevadensis, Meek. 



Plate 8, figs. 2, 2 a, 2 b, 2c, 2d, 2 e. 



Shell of medium size, with a general elongate-subovate form, as seen in 

 adult examples, which are much produced anteriorly; hinge less than the 

 greatest breadth. Ventral valve very gibbous, strongly arched, the curve 

 regularly increasing from the produced front to the beak, provided with a 

 rather deep mesial sinus, which extends from near the beak to the front; gib- 

 bous central region comparatively narrow, and, on each side of the sinus, 

 rounded and falling off laterally very abruptly, especially to the ears; beak 



