162 PALEONTOLOGY OF THE EUREKA DISTRICT. 
sial fold having a shallow median sinus; beak short, incurved. Ventral - 
valve most convex towards the beak, flattening out in the broad mesial sinus; 
the latter is marked by a low rounded median plication. 
Surface of dorsal valve marked by a single low plication on each side 
of the mesial fold that corresponds’to a shallow sinus each side of the pli- 
cation bordering the large mesial sinus of the ventral valve; a few fine con- 
centric lines of growth are visible by the aid of a strong lens. 
Dimensions: height of largest specimen in the collection, 9™"; width, 
12™™; convexity of the two valves united, 8™™. 
On a shell having a height of 4°", a transverse diameter of 5", and a 
depth of 2.25™™ for the two valves, the mesial sinus and fold are shown only 
by an undulation in the front margin, the valves are subequal in convexity 
and the beak of the dorsal valve is scarcely elevated above that of the ven- 
tral. With the increase in the size of the shell, the features of the adult. 
gradually appear until, in the largest specimens, there is very little variation 
in form except a greater or less convexity as the shell is more or less trans- 
verse. 
Of American species Pentamerus subglobosus M. & H. (Geol. Illinois, vol. 
iii, p. 429, plate xiii, figs. 5 a—c), approaches this species in size and form, but 
the numerous plications and different convexity of the valves distinguish 
them. Pentamerus biplicatus Schnur, of the Middle Devonian of England 
(Davidson, Mon. Brit. Foss. Brach., vol. iii; p. 73), may also be compared 
with P. Lotis. 
It is with considerable hesitancy that this Upper Devonian shell is 
referred to a new species, as it may possibly be a form descended from P. 
comis, of the Lower Devonian of the Eureka District. No young shells of 
the latter species have been found that we can compare with the earlier 
stages of growth of P. Lotis, but in the larger form of the two species de- 
cided differences appear, as may be seen by a comparison of the figures 
representing them. 
Comparing P. Lotis with smooth specimens of P. comis, from the Upper 
Devonian horizon at Rockford, Iowa, we find a decided resemblance between 
them, but not sufficient to unite them as one species. 
Formation and locality—Upper Devonian, on the west side of Apple- 
gate Cation, White Pine Mining District, Nevada. 
