72 GEOLOGY OF THE EUEEKA DISTRICT. 



Tlie Upper Helderberg horizon of the New York series is represented by thirty-eight 

 species common to it and the lower portion of the Devonian of the Eureka district; 

 the Chemung group of the same by sixteen species; of tlie Hamilton species of New 

 York twenty-three are distributed through the lower portion of the Eureka Devonian 

 limestone and eighteen species in the middle and upper i)ortions, but not in sucli a 

 manner as to distinguish a middle division corresponding to the Hamilton formation 

 of New York. Of strictly Hamilton species in New York, twenty -three are found, of 

 which eleven are in beds a little below the summit, and twelve just above the base of 

 the formation. 



Eleven species not known in New York are common to both the 

 Great Basin and Iowa, thus emphasizing the faunal relations between the 

 corresponding horizons in the Cordillera, the Mississippi Valley and the 

 Appalachians. 



While the fauna at Eureka is rich and varied, both in genera and 

 species, remains of Devonian fishes appear to be restricted to a single 

 ctenacanthus-like tooth. Mr. S. F. Emmons, while engaged on the Fortieth 

 Parallel Exploration, brought in a small tooth of the genus Cladodm 

 from the western entrance to Emigrant Canyon, in the Tucubit Moun- 

 tains north of Humboldt River. These two single specimens, collected 

 at widely separated points, are all that is known of Devonian fishes from 

 Central Nevada, although from Northern Arizona, in the Kanab Canyon, 

 Mr. C. D. Walcott' obtained abundant evidence of the presence of placo- 

 ganoid fishes from Devonian beds, which were represented by only 100 

 feet of strata as against 8,000 feet in Nevada. 



Corals occur tlu-oughout the Nevada limestone and certain species 

 present a wide vertical range. Among these Stromatopora are known from 

 base to summit, and in one or two horizons they are found in such profusion 

 as to characterize the strata by the peculiar weathering-out of the imbedded 

 silicified corals. In the siliceous limestone of the Upper Devonian, fragments 

 of Syringopora associated with Stromatopora are occasionally abundant when 

 all other species are wanting. The bedded limestone on both sides of the 

 Yahoo Canyon offer favorable conditions for the j^reservation of these 

 foi'ms. Prior to the survey of the Eureka District the Lamellibranchiates 

 were poorly represented from the Great Basin. To a meager list almost 



' Am. Jour. Sci. Sept., 1880. 



