95 



ACROTRETA, PAPILLATA, PI. Ill, figs. 2 /. 



Bull. Nat. Hist. Sec. of N. Bruns'k, vol. iv, p. 390, pi. xv,figs. 2 a-f. 

 (Calcareo-)corneous valves moderately arched, nearly orbicular, ventral Acrotrefca 

 valve with a moderately elevated uinbo, one-fifth from the back of the 1 ' a i' lllata - 

 valve ; the back of the valve somewhat concave toward the umbo, but 

 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. 



The dorsal valve is moderately arched, the slope being steepest toward 

 the umbo, which is but slightly raised. On each side of the umbo flat- 

 tened slopes run along the sides of the valve in the posterior half. There 

 is a shallow median sinus on the beak 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 the 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, 2J mm. long, 2 to 3 mm. wide, and 1| mm. high. 

 Dorsal, as the ventral, except th t the height is about f mrn. 



Horizon and locality. E. 1 d, the Gregwa shale of the Etcheminian at 

 Dugald brook, Escasonie, (C. B.) N.S. Common. It occurs also in 

 Assise E. 1 c. 



VAR. LATA, PI. Ill, figs. 3 a-c. 



Bull. Nat. Hist. Soc. of 1ST. Bruuswick, vol. iv, p. 391, pi. xv, figs. 3 a-c. 



In examples from Boundary brook the form of the callus in the interior Variety 

 of the ventral valve varies from a perfectly circular elevation to one that a 



