ADDITIONAL NOTES ON THE CAMBRIAN OF CAPE BRETON. 
391 
The dorsal valve is moderately arched, the slope being steepest 
toward the umbo, which is but slightly raised. On each side of the 
umbo flattened slopes run along the sides of the valve in the posterior 
half. There is a shallow median sinus on the back of the valve, which 
widens toward the front. Interior — Under the beak is a boss from 
which a median septum runs forward, that forks about a fifth of the 
length of the valve from the hinge line ; from the space between the 
forks, at a third of the length of the valve from the hinge, the median 
septum reappears, widens, and terminates at a point nearly a third 
from the front of the valve. On each side of umbo are pits of the 
cardinal muscles, and outside these, in advance of them, and near the 
margin, are large scars of the lateral muscles. 
The margins of both valves are flattened and thickened. 
Sculpture. — This shell has a dull, minutely granulated surface, 
across which run narrow ridges concentric to the umbo, widely spaced 
in the middle of the shell, more closely arranged toward the margin, 
and closely crowded and narrow, on each side toward the hinge. 
Size, — Ventral, 2^ mm. long, 2J to 3 mm. wide, and 1 J mm. high. 
Dorsal, as the ventral, except that the height is about ^ mm. 
Horizon and locality. — E. 1 d, the Gregwa shale of the Etche- 
minian at Dugald Brook, Escasonie, (C. B.) N. S. Common. It 
occurs also in Assise E 1 c. 
Var. pi. XV, figs. 3 a-c. 
In examples from Boundary Brook the form of the callus in the 
interior of the ventral valve varies from a perfectly circular elevation 
to one that is somewhat squared at the sides ; the groove outside of 
the callus is sometimes indistinct. On each side of the foramen is 
sometimes a short, sharp furrow directed forward. In the dorsal 
valve the depressed posterior lateral slopes and the somewhat flattened 
anterior slope gives the valve a triangular appearance. The interior 
shows a pit at the hinge area, which is narrow, and thence a narrow 
median ridge runs nearly to the middle of the valve. An inconspicuous 
lateral branch is thrown off on each side of the median ridge. 
Acrotreta papellata-prima, n. mut. PI. XV, figs. 1 a-c. 
Only the ventral valve of this form is known. This is wider than 
long, tumid, with the cardinal area vertical. Interior. — In this the 
