Camhrian Rocks in Cape Breton. 449 



ing, but the anterior marginal curve is more oblique to 

 the hingeline, the ventral angle being opposite to the post- 

 erior half of the cardinal line ; this (2 mm. long) is consi- 

 derably more than half of the length of the valve. 



The anterior cardinal curve is angulated at each end ; 

 the posterior is a third longer than the anterior. The 

 anterior marginal curve is straightened and is consider- 

 ably longer than the posterior, which is strongly arched 

 outward. A narrow thread-like marginal fold is visible 

 in some places. A thickened band within the margin, in 

 some places shows slight protuberences. 

 Smilpture. In most examples the surface is scabrous from 

 corrosion, but some show traces of a minute pitting, and 

 near the margins of the valve these pit form continuous 

 rows, or furrows. The mould of the interior shows a 

 smooth surface having minute punctures. The muscle 

 scar behind the ocular tubercle, is distinct on the mould. 



Size. Length 3| mm ; width 3 mm ; depth of a valve 



1 mm. 



Horizon and locality. In Assise E. 1. f/., Lower Etchemi- 

 nian, at Boundary Brook, Common. 



The following are measurement of several valves from 

 this locality, 

 left valve, length 3^ mm, width 2| mm, depth 1 mm. 



2 " " " 31 " " 3 



31 " " 2f 

 31 " " 2f 



carapace " 3f " " 2f " 



Three forms which may be classed as varieties of this 

 species are the following. 



Var. BENEPUNCTA, n. var. PI. I. fig. 16. 



Anterior cardinal curve longer than the posterior ; ante- 

 rior marginal curve straightened and the greatest width 

 of the valve posterior to the middle. Hinge-line nearly 

 half of the length of the valve (21 mm). Ocular tubercle 



