CABBONIFEROUS SPECIES. 79 



regarded by reliable authorities as being varieties of Sowerby's species. 

 The best specimen in the collection is represented by the figs. 4, 4 a, on 

 plate 8. This is more gibbous in the iimbonal region, and has larger cos- 

 t3B than the most characteristic examples of P. longispinus, while it shows no 

 traces of the mesial sinus usually seen in the same. In these respects, 

 however, it agrees more nearly with a form illustrated as a variety of that 

 species by Mr. Davidson's figs. 19, 19 a, plate xxxv, of his Monogr. Brit. 

 Carb. Brach. It is true that it does not show the angular projection of the 

 anterior margin seen in Mr. Davidson's figure, but its anterior margin is 

 broken. Some of the other more imperfect specimens, however, show the 

 usual mesial sinus, and the smaller costse, exactly as in characteristic exam- 

 ples of P. hngispinus. Generally, the surface is exfoliated, and the spines 

 broken away; but, in a few instances, some remaining traces of their bases 

 can be seen, while fragments of comparatively stout spines, like those of 

 Sowerby's species, occur closely associated in the same matrix. 



Locality and position. — Fossil Hill, Wliite Pine District, Nevada; in light- 

 colored Carboniferous limestone. 



EHYXCHONELLID^. 



Genus LEIORHYNCHUS, Hall. 

 Leiorhynchus QUADRicosTA'i'us, Vanuxem ? (sp.). . 



riato3, figs. 9, 9 a, 9&. 

 Ortliis quadricostatiis, Vanux. (1842), Geol. Eep. Third Dist. N. Y., 18G. 

 Leiorhynchus quadricostatus, Hall (1843), Eegent's Thirteenth Eep., 8G; and Pal. K. Y. 

 iv, 357. 



Shell trigonal-subovate, or subcircular, very thin, and apparently com-' 

 pressed; posterior lateral margins somewhat straightened, and diverging 

 from the beaks at about a right angle; anterior lateral margins rounding to 

 the front, which is generally a little protuberant in the middle, but some- 

 times rounded, or even faintly sinuous in outline. Dorsal valve apparently 

 more convex than the other, and provided with a depressed mesial fold, which 

 extends little farther than the middle, and bears four, or rarely five, low, 

 rounded costae, which do not reach the umbonal region; lateral surfaces 

 smooth, or only showing very faint traces of a few undefined radiating 

 costse. Ventral valve with a shallow mesial sinus, corresponding to the fold 

 of the other valve, and bearing three, or rarely four, obscure costse, while 



