FOSSILS OF THE CARBONIFEEOUS. 



15 



The Carboniferous form is associated with Productus Cora, P. semire- 

 ticulatus, P. muricatus, P. elegans, Spirifera Eockymontana, S. camerata, etc. 



Formation and locality. Lower portion of Lower Carboniferous lime- 

 stone, in canon directly south of a small conical hill on east side of Secret- 

 canon-road Canon, Eureka District, Nevada. 



Genus SPIKIFERA Sowerby. 



Spirifera trigonalis Martin (Sp.). 

 Plate xviii, fig. 11. 



Conchyliolites anomites trigonalis Martin, 1809. Pet. Derb., tab. xxxvi, fig. 1. 



Spirifer trigonalis Sowerby, 1820. Min. Con., tab. 265, fig. 1 (not 2 and 3). 



Spirifera trigonalis, var. a, McCoy, 1855. Brit. Pal. Foss., p. 423. 



trigonalis Davidson, 1858. Brit. Carb. Brachiopoda, p. 29, pi. v, figs. 25- 



34; 35-37? 



lisulcata Davidson, 1861. Ibid. Mr. Davidson gives the synonomy up to the 

 date of his publication, 1853, also that of Spirifera crassa De Koninck, Spiri- 

 fera grandicostata, and S. transiens McCoy, which he regards as forms of this 

 widely varying species. 



Spirifer increbescens Hall, 1S5S. Geol. Surv. Iowa, vol. i, pt. 2, p. 700, pi. xxvii, figs. 6a-i. 



After a most careful study of a large series of specimens of this species, 

 Mr. Davidson has united all the forms previously placed under the species 

 mentioned in the list of synonyms as variations of a single very variable 

 species and, in speaking of the winged, simple form and the transversely 

 oval, rounded, thickened variety crassa, says: "The notion of both being 

 the modification of a single species will, to the generality of observers, 

 appear absurd ; still if we find every variation connecting these extremes, 

 are we to refuse the evidence of our eyes and senses, and to create as many 

 species as we possess specimens?" An examination of Mr. Davidson's beauti- 

 ful illustrations cannot fail to convince one of the specific relationship of 

 the different forms, and I have no doubt that the Nevada shell before me is 

 one of the varieties of this widely-distributed species. 



In the Mississippi Valley it is found in the Chester limestone of the 

 Lower Carboniferous formation of the State of Illinois, and described by 

 Professor Hall as Spirifera increbescens. Mr. Davidson states that it is 

 abundant in England, Ireland, and Scotland, and that on the Continent it is 

 found in Belgium. 



