140 PALEONTOLOGY OF THE EUREKA DISTRICT. 



much more transverse than others and the relative length of the hinge line 

 varies. 



The surface is usually smooth, owing to the condition of preservation 

 of the shell. Examples show, however, that it was marked in some instances 

 by obscure radiating plications, concentric striae about I" 1 distant, also fine 

 radiating interrupted striae. These vary in different shells and in some are 

 entirely absent. 



The variety differs from the typical specimens of S. (M.) glabra in having 

 the greatest transverse diameter higher up on the shell, and the mesial fold 

 and sulcus usually more angular. It also averages smaller in size, the larg- 

 est specimen having a height of 30 mm , with a breadth of 38 mm , and the aver- 

 age about 25 mm by 30 mm . 



Professor Hall compares S. (M.) glabra and S. (M.) Icevis of the Devonian 

 of New York, and more recently the latter has been considered a variety 

 of S. (M.) glabra, 11 a reference that I would concur in, placing it, the Ne- 

 vada variety, and Mr. Meek's S (M.} Franklini, from British America, as 

 the three known representatives of the Carboniferous species in the Devo- 

 nian rocks of the eastern and western side of the continent. 



Formation and localities. Upper portion of the Devonian limestone, on 

 the ridge east of Yahoo Canon, and on the south side of The Gate, north- 

 west of Eureka, Eureka District, Nevada. 



The above was written in 1881, as the result of the study of the spec- 

 imens then in the collection. In the summer of 1882 I returned to the 

 locality and obtained a number of good specimens, some of which have 

 faintly defined radiating plications, thus showing that those represented on 

 fig. 14&, plate xiv, were not the result of irregular exfoliation of the outer 

 shell, but a portion of the original surface. An additional figure, plate iii, 

 fig. 5, is given of a less transverse specimen than that represented on plate 

 xiv, figs. 14, 14 a, b. This is of the same character as that of S. (J) undifera, 

 from the Lower Devonian, plate iii,, figs. 6, 6 a. The relations of S. (M.) 

 glabra and S. (M.) undifera will be spoken of in the description of the latter 

 species. 



11 H. S. Williams' Life History of Spirifera Icevis, Ann. New York Acad. Sciences, vol. ii, 1881. 



