390 
BULLETIN OF THE NATURAL HISTORY SOCIETY. 
Dugald Brook, Escasonie (C. B.), N. S. Also a doubtful ventral 
from E. 3 /! at Gillis, Indian Brook, Escasonie. Scarce. 
This mutation is distinguished from the type and from mutation 
prima by its robust form, and from proavia^ the type, also by the 
possession of a thickened callus. From the mutations and type of 
A. signata by the narrowness of its callus. 
Of the two species of Acrothyra herein described, signata was 
found specially to characterize the lower half of the Lower Etcheminian 
fauna, being found most abundant in the middle measures of this set 
of beds. It is not, however, limited to these measures, but by muta- 
tions is sparingly represented in the upper part of this lower fauna. 
Acrothyra proavia, on the contrary, has been found only in the 
Upper Fauna, and mostly in its higher part, where some layers are 
crowded with thousands of these little shells. 
ACROTRETA. Kutorga. 
While this genus appears as a contemporary of Acrothyi-a in the 
earliest Basal Cambrian, it seemingly lived on after the latter had 
passed away. But throughout the Coldbrook and lower Etcheminian 
measures, it is (piite subordinate in numbers to Acrothyra, and we 
have not found it at all in the upper Etcheminian. Throughout the 
true Cambrian, in the Acadian Provinces, however, these conditions 
were reversed, for, with the doubtful exception of LingulcUa (Acro- 
thyra ?) inj^lata of the Protolenus fauna, an undoubted example of the 
genus xVcrothyra is unknown to me above the Etcheminian horizon, 
and Acrotreta has full possession of the field. 
Acrotreta papillata, n. sp. PI. XV, figs. 2 a-f. 
(Calcareo-)corneous valves moderately arched, nearly orbicular. 
A'entral valve with a moderately elevated umbo, one-fifth from the 
back of the valve; the back of the valve somewhat concave toward 
the umbo, Vjut convex toward the front margin. There is a concave 
pseudo-deltidium, and the side slopes of the hinge area are convex. 
Interior — In this the visceral callus is short, sub-circular, and marked 
at the middle by a deep circular pit ; at its sides, obscure, short, 
straight, diverging grooves are usually seen within the circular groove 
that surrounds it, 
