154 PALEONTOLOGY OF THE EUREKA DISTRICT. 



Dorsal valve gibbous, sloping somewhat abruptly to the margins of the 

 opposite valve; mesial fold prominent in the younger shells, and marked 

 with three or four plications that extend up to the upper third of the valve. 

 In older specimens the fold is scarcely traceable above the middle of the 

 valve, and the plications, six or eight, are usually short and confined to the 

 lower part. 



Surface of younger shells with obscure plications on the sides and 

 stronger, depressed, rounded plications on the mesial fold and sinus. Con- 

 centric lines of growth mark the upper part of each valve. The surface 

 of the older shells is smooth with the exception of the plications on the 

 mesial fold and sinus and a few lines of growth. 



This is a somewhat variable species in its different stages of growth. 

 The younger shells are subcuboidal and of the type of Wiynclionella venus- 

 tula Hall (Pal. N. Y., vol. iv, p. 346, pi. liv, A, figs. 24-43) of the Tully 

 limestone of New York, while the adults are subglobose, with the ventral 

 valve less convex, and the angle formed by the union of the sides of the 

 two valves more acute. The two extremes of growth might readily be 

 separated as distinct in their specific characters. The species is of the type 

 of Rhynchonella Emmonsi Hall and Whitfield (Gleol. Expl. Fortieth Par., vol. 

 x, p. 247, pi. iii, figs. 48) from the Devonian limestone of White Pine 

 Mountains, Nevada; but it is specifically distinct from that species, and also 

 H. venustula and R. cuboides Sowerby (sp.) (Davidson, Mon. Brit. Foss. 

 Brach., vol. iii, pt. 6, p. 65, pi. xiii, figs. 1721) of the same group of species, 

 although more or less resembling them in some of its phases of develop- 

 ment. 



The above was written before I had seen the description and illustra- 

 tion of Mr. Meek's species, M. castanea, from the Devonian limestone in the 

 Mackenzie River Basin. A careful comparison of the Nevada specimens 

 with Mr. Meek's description and figures, leads to the opinion that R. castanea 

 is one of the variations of the species as it occurs in the Eureka District, 

 and that the latter form should be united with it under the same specific 

 designation, as also the shell illustrated by Mr. Meek as probably a distinct 

 species (Trans. Chicago Acad. Sci., vol. i, plate xiii, fig. 10a, fc), but which is 

 the more convex or subcuboidal form of the species. 



