88 GUBGAliliOXlFEHOUS I'lililOl). 



Spirifer striatus Mtirtiu, sp. 

 Plato V, lig. 10 a. 



Associated with characteristic Subcarboniferous forms at Mountain 

 Spring, old ]\Iormon road, Nevada, some specimens of the well and widely- 

 known Spirifer striatus were obtained. They are all more or less imperfect, 

 but their identity with that species seems unquestionable. The strias which 

 mark the surface are regular and nearly uniform in size, and show no inclina- 

 tion to become fasciculated as those of S. cameratus always do to a greater or 

 less degree. The collections also. contain specimens, apparently of this 

 species, from a higher geological horizon, which are noticed upon another 

 page, among the Coal-Measure species. 



Varieties of S. striatus have been described under different sjoecitic 

 names from the Subcarboniferous rocks of the United States; but when com- 

 pared with authentic specimens from the Carboniferous rocks of Eui'ope, I 

 am not able to discover differences of specific value. In view of this fact, I 

 have less hesitation in referring our specimens to S. striatus than I other- 

 wise would have. 



Spirifer extenuatus H:ill. 



Plate V, fig. 9 a, b, c, and d. 



Spirifer extemmtus Hall, 1858, Geology of Iowa, i, pt. ii, 520. 

 Syringothyris Halli Wincbell, 18G3, Proc. Acad. Nat. Sci. Pliila., 8. 



Shell of medium size, much extended at the hinge-line; length from 

 hinge to front usually a little less than the height from the ventral beak to 

 the dorsal umbo. ' 



Ventral valve capacious, subpyramidal, having a moderately deep 

 sinus, which is well defined from front to beak, and rapidly widening toward 

 the front; beak extended, pointed, its point a little incurved over the area; 

 sides sloping from the beak to the hinge-extremities with little or no con- 

 vexity of outline, and also little or no convexity to the front and lateral 

 margins ; area large, triangular transversely striated, flat or distorted a little 

 by the t^visting of the beak, and its inclination more or less toward the dex- 

 ti'al side of the shell; fissm-e of the area triangulai-, nearly twice as high as 

 it is wide at the base; each of its lateral border-angles truncated by a shal- 



