93 



forward on each side of the front slope of the ventral valve, may indicate 

 the position of vascular trunks. 



Interior. The dorsal valve has inside a median and two lateral ridges. 

 On some valves the median ridge extends only so far as to divide the pits 

 of the cardinal muscle ; in others it extends to the middle of the valve. 

 A pair of median pits are sometimes visible near the end of the median 

 septum, one on each side of it. 



ACROTHYRA PROAVIA-PRIMA, PI. II, figs. 4 O-f. 



Bull. Nat. Hist. Soc. of N.B. vol. iv, p. 389, pi. xiv, figs. 4 a-/. 



Ventral valve triangular- ovate, about twice as long as wide, prolonged Mutation 

 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 ; eak, 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 edge 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 mno. Dorsal valve, 2 mm. in each diameter ; depth, 

 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 appar- 

 ently of this form, 3x2 mm., from E. 3 /.', 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. 



