FOSSILS OF THE DEVONIAN. 123 
These features are usually more or less obscured by the complete or partial 
exfoliation of the shell, even if it has escaped being worn or macerated 
previous to being embedded in the sediment. The varying conditions of 
preservation, if in a pure limestone, calcareous or argillaceous shale, or in 
an arenaceous rock, also materially complicate the question of the identity 
of species from various horizons and widely separated localities. The 
identifications between the Nevada and New York specimens of the same 
species were made with the above conditions in view and on a direct com- 
parison with authentic specimens in the Hall collection at the American 
Museum of Natural History in New York City, most of which were used 
in the original diagnoses of the various species. 
Chonetes hemispherica Hall. 
Chonetes hemispherica Hall, 1857. Tenth Ann. Rep. N. Y. State Cab. Nat. Hist., p. 116, 
and Pal. N. Y., vol. iv, p. 118, pl. xx, fig. 6, 1867. 
hemispherica Billings, 1861. Can. Jour., vol. vi, p. 349. 
Nicholson, 1873. Pal. Prov. Ontario, p. 75. 
Compare Strophomena gibbosa Conrad. 
The types of this species are from the Schoharie grit and Corniferous 
limestone of the Upper Helderberg Group of New York. It is a strongly 
marked form, and with the exception of the closely allied C. arcuata, from 
the Corniferous limestone, it is not likely to be confused with any other 
species from the Devonian. The Nevada specimens have the same ventri- 
cose or subhemispheric ventral valve, with its prominent umbo rising above 
the hinge line. The surface is largely exfoliated from all the specimens in 
the collection, but sufficient remains to exhibit the characteristic radiating 
strie. As far as yet known it is confined to the Lower Devonian, and is 
associated with other well-defined Upper Helderberg forms, viz: Productus 
(P.) navicellus, P. (P.) truncatus, Spirifera raricosta, ete. 
Among the collections of the Fortieth Parallel Exploring Expedition 
there is a specimen of this species from the summit of Cave Cation, Pitton 
Mountains, Nevada. 
Formation and locality—Lower horizon of the Devonian limestone, 
Lone Mountain, 18 miles northwest of Eureka, Nevada. 
