FOSSILS OF THE CAMBRIAN. 15 
in by successive additions of the shell, so that the general circular outline 
of the shell has scarcely been broken by the very slight truncation of the 
posterior margin. The additions to the posterior margin of the false area 
are crowded together as little transverse ridges with fine stria between. 
The outer portion of the shell is largely exfoliated, but traces of ra- 
diating striz and concentric lines of growth are shown. The latter on the 
outer portion appear to have been continuous with those crossing the 
pseudo-deltidial area From each side of the apex a narrow, elongate, 
muscular scar gently diverges, extending towards the central portion of the 
shell. Transverse diameter, 7.5™; longitudinal diameter, 6.75"; eleva- 
tion or convexity, about 0.75™. 
The reference of this species to the genus Acrothele is made with reser- 
vation, Its depressed form and eccentrically-perforated apex are similar, 
while the false area and the muscular scar are different. It is, however, 
more closely allied to Acrothele than with any genus with which we are 
acquainted, 
Formation and locality—Cambrian, Prospect Mountain Group; upper 
beds of the Secret Canon shale, on the east side of Secret Canon, Eureka 
District, Nevada. 
Genus SCENELLA Billings. 
Scenella ? conula, n. sp. 
Plate ix, fig. 6. 
Shell small, conical; apex subcentral, apparently with a tendency to bend 
a little to one of the sides on which there is a very faintly-indicated line run- 
ning from the apex to the margin. Aperture ovate. Surface smooth to 
the unaided eye; it shows fine concentric striae and a few obscure lines of 
growth when examined by the aid of a strong magnifying glass. 
Dimensions.—Greater diameter, 3"; lesser diameter, 2.25": eleva- 
tion, about 1.75™™. 
This is a small Metoptoma-like shell, the generic reference of which is 
provisional, as it differs from the type of the genus, Scenella reticulata, in not 
having a carina running from the apex to the margin; the curvature of 
