FOSSILS OF THE LOWER SILURIAN. 7& 



naria, etc. Under such circumstances we prefer to refer all such species 

 to described forms unless they show good, specific differences, or a dif- 

 ference in their geologic horizon that renders their occurrence extremely 

 improbable. 



Formation and locality. About 1,000 feet down in the Pogonip Group, 

 north slope of White Mountain, Eureka District, Nevada. 



Genus MURCHISONIA D'Archiac and De Yerneuil. 



Murchisonia Milleri Hall? 

 Plate i, figs. 12, 12 a, b. 



Murchisonia bicincta Hall, 1847. Pal. N. Y., vol. i, p. 177, pi. xxxviii, figs. 5, 5 a-h. 



Mcincta Salter, 1858. Canadian Organic Remains, dec. i, p. 19, pi. iv r 



figs. 5, 6, 7. 



bicincta M. & W. ?, 1868. Geol. Surv. Illinois, vol. iii, p. 317, pi. iii, fig 4. 

 Milleri Hall, 1877. Cat. Auier. Pal. Fossils, Miller, p. 244. 

 Not Murchisonia bicincta McCoy, 1844. Syn. Sil. Foss. Ireland, p. 16. 



On comparing specimens of Murchisonia Milleri, from Middleville, New 

 York the typical locality of the species, with the Nevada shell, certain differ- 

 ences appear that would serve to separate the form from Nevada as a 

 distinct species from M. Milleri if the range of variation in the specimens 

 from the two localities did not afford examples from each that are specifi- 

 cally identical. As compared with the majority of the New York and 

 Canadian specimens, the Nevada shell is more depressed, the aperture less 

 elongate, and the carina above and below the peripheral carina less strongly 

 marked, differences, however, that are not apparent when comparing with 

 some shells from the typical locality at Middleville; with this in view, and 

 the fact that a certain range of variation may be allowed for changes pro- 

 duced during the geographic distribution of the species, I think we are 

 justified in identifying the Nevada shell with M. Milleri. It is associated 

 with common Trenton limestone fossils, Orthis tricenaria, O. perveta, Raph- 



istoma Nasoni, lllcenus crassicauda, and other species indicating the Trenton 

 fauna. 



Formation and locality. Lower Silurian, near the summit of the Pogo- 

 nip limestone of Pogonip Ridge, White Pine District, Nevada. 



