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 wera used 

 in the original diagnoses of the various species. 



Chonetes hemispherica Hall. 



Clwnttes hemispherica Hall, 1857. Tenth Ann. Rep. N. Y. State Cab. Nat. Hist., p. 116, 



and Pal. N. Y., vol. iv, p. 118, pi. 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 

 striae. As far as yet known it is confined to the Lower Devonian, and is 

 associated with other well-defined Upper Helderberg forms, viz : Produdus 

 (P.) navicellus, P. (P.) truncatus, Spirifera raricosta, .etc. 



Among the collections of the Fortieth Parallel Exploring Expedition 

 there is a specimen of this species from the summit of Cave Canon, Pinon 

 Mountains, Nevada. 



Formation and locality. Lower horizon of the Devonian limestone, 

 Lone Mountain, 1 8 miles northwest of Eureka, Nevada. 



