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 Rhynchonella venus- 
tula Hall (Pal. N. Y., vol. iv, p. 346, pl. 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 (Geol. Expl. Fortieth Par., vol. 
x, p. 247, pl. iii, figs. 4-8) from the Devonian limestone of White Pine 
Mountains, Nevada; but it is specifically distinct from that species, and also 
R. venustula and R. cuboides Sowerby (sp.) (Davidson, Mon. Brit. Foss. 
Brach., vol. iii, pt. 6, p. 65, pl. xiii, figs. 17-21) 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, R. 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 2. 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, b), but which is 
the more convex or subeuboidal form of the species. 
