FOSSILS OF THE LOWER SILURIAN. 19 
naria, ete. 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’Archiae and De Verneuil. 
Murchisonia Milleri Hall? 
Plate i, figs. 12, 124, b. 
Murchisonia bicincta Hall, 1847. Pal. N. Y., vol. i, p. 177, pl. xxxviii, figs. 5, 5 a-h. 
bicineta Salter, 1858. Canadian Organie Remains, dec. i, p. 19, pl. iv, 
figs. 5, 6, 7. 
bicincta M. & W. ?, 1868. Geol. Surv. Illinois, vol. iii, p. 317, pl. iii, fig 4. 
Milleri Hall, 1877. Cat. Amer. 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 J/. 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 MW. Milleri. It is associated 
with common Trenton limestone fossils, Orthis tricenaria, O. perveta, Raph- 
istoma Nasoni, Illenus crassicauda, and other species indicating the Trenton 
fauna. 
Formation and locality—Lower Silurian, near the summit of the Pogo- 
nip limestone of Pegonip Ridge, White Pine District, Nevada. 
