Cambrian Rocks in Cape Breton. 445 



Breton Etcheminiaii rocks and, if the following mutations 

 are properly referred to it, extends through them in vary- 

 ing forms and sizes. 



In this typical form the hinge is more than three-fifths 

 of the width of the valves. The posterior cardinal curve 

 is long and straight, the margin bears a thread-like mar- 

 ginal fold on the left valve and there is a narrow obscure 

 furrow, within the margin along the anterior and posterior 

 marginal 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 extending a 

 short distance below it. There is also a short, obscure 

 ridge extending obliquely downward from the ocular tu- 

 bercle 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 

 appearance of parallel ridges has been produced. 



Size. Length 4f mm. Width 4 mm. Depth of each 

 valve 1| mm. 



Horizon and locality. This is of the Lower Etcheminian 

 Fauna, and occurs in Assise E. 1 . d. at Duc^ald Brook. Esca- 

 sonie, N. S. — Erequent. 



The following measurements exhibit some variations in 

 size. 



A right valve, length 4| mm, width 4| mm, depth 1|- mm. 

 Another " " 4 " " 4f " " 1* " 



A left " . " 4 " '' 4f ■ " " 11 " 



Mutation maxima, n. mut. PL I., figs. 9 a, &. 

 This is the largest form of the species observed ; it is 

 33 



