157 



obscure furrow, within the margin along the anterior and posterior mar- 

 ginal curves. The hinge margin is thickened, and a tubercle marks the 

 posterior end of the hinge line (of at least the right valve). The ocular 

 tubercle is distinct in each valve, behind which is a shallow furrow extend- 

 ing a short distance below it. There is also a short, obscure ridge extend- 

 ing obliquely downward from the ocular tubercle toward the lower margin 

 of the valve. 



The greatest convexity of the valve is two-fifths below the hinge line, 

 and the slope to the anterior margin is steep. 



Sculpture, In all the forms of this species obtained, the sculpture has 

 been obscured by corrosion, but remains of the cortex that have escaped 

 this destructive change show a pitted surface. By a linear arrangement 

 of the pits along the anterior and posterior slopes of the valves an appear- 

 ance of parallel ridges has been produced. 



Size. Length 4f mm. Width 4 mm. Depth of each valve 1J mm. 



Horizon and locality. This is of the Lower Etcheminian Fauna, and 

 occurs in Assise E. 1. d. at Dugald brook, Escasonie, N.S. Frequent. 



The following measurements exhibit some variations in size. 



A right valve, length 4 mm., width 4^ mm., depth 1 mm. 

 Another " " 4 " 4| " " 1$ " 

 A left " " 4 " " 4f " " 1J " 



Mutation MAXIMA. PI. XII., figs. 9. a, b. 



Can. Rec. Sci. Montreal, Vol. VIII., p. 445, 1902. 



This is the largest form of the species observed ; it is more rounded at Mutation 

 the two ends and below, than the type, and is flatter, but is of the same described, 

 general form. 



The hinge-line is two-thirds of the length of the valve. The posterior 

 cardinal curve is angulated. There is an obscure furrow behind the ocular 

 tubercle, extending half across the valve ; a low ridge extends along the 

 anterior margin, a little within it, and a fainter ridge along and near the 

 posterior cardinal curve ; a narrow marginal furrow is visible along the 

 anterior marginal curve ; an obscure row of tubercles extends along and 

 near the anterior half of the hinge of line. 



Sculpture. Surface pitted, the spaces between the punctures becoming 

 anastomosing ridges near the margin, and presenting ridges on the pos- 

 terior half of the valve, sub-parallel to the margin. 



Size. Length 6 mm., width 5 mm., depth of a valve If mm. 



