ADDITIONAL NOTES ON THE CAMBRIAN OF CAPE BRETON 389' 
Acrothyra proavia-prima, n. mut. PI. XIV, figs. 4 a-f. 
Ventral valve triangular-ovate, about twice as long as wide, pro- 
longed into a long pointed beak, and rounded and bent down in front. 
Interior. — This exhibits a long narrowly tapering callus originating in 
the beak at the foramen ; the callus is a third of the length of the valve, 
or more. In front of the callus is a shallow transverse depression, 
marking the position of the central group of muscles. 
The dorsal valve is nearly circular, projecting at the back, where 
there is a somewhat low beak, and rounded down more at the front 
than the sides. Interior . — A median septum is visible, dividing the 
pits of the cardinal muscles. A shallow median ridge traverses the 
middle of the valve, which is flattened at each side near the hinge. 
The edges of both of the valves are flattened and thickened, also the 
apical third of the ventral valve is thicker than the middle of that 
valve. 
Sculpture . — This consists of a very fine granulation, with frequent, 
thread-like, concentric ridges. 
Size. — Ventral valve : length, 3 mm.; width, 2 mm. ; height of 
the cardinal area, 2 mm. Dorsal valve, 2 mm. in each diameter ; 
depth, J mm. 
Horizon and locality. — E 3 a = base of the upper Etcheminian 
shale at Dugald Brook, Escasonie (C. B.), N. S. Frequent. In this 
rock the ventral valves of Acrothyra lie flat on the layers. Also a 
valve apparently of this form, 3x2 mm., from E. 3 /1, occurs at Gillis 
Brook, a branch of Indian Brook, Escasonie. 
This mutation is distinguished from the type by its greater size 
and by the possession of a thickened callus. 
Acrothyra proavia-crassa, n. mut. PI. XIV, figs. 5 a-c. 
Only the ventral valve known. This is short, tumid and conical. 
Interior . — This possesses a narrow callus, four or five times as long as 
wide, and nearly a third of the length of the shell. At the front of 
the callus are two small oval scars divided by a faint septum. The 
callus is concave and extends back nearly to the beak. 
Sculpture . — Some fragments of the surface which are preserved' 
show fine, close set, concentric ridges. 
Size. — Length, 2|^ mm.; width, 2 mm.; height, mm.? 
Horizon and locality . — Lower layers of the assise E. 3 e. at 
