Cambrian Rocks in Cape Breton. 453 



Horizon and locality. Assise E. 3. e., Upper Etcheminian, 

 at Dugald Brook. 



Additional material shows much better the characters 

 of this species. In this the valves have been of a more 

 distinctly oval form than in those of the preceeding spe- 

 cies, the cardinal curves being rounded so that the 

 straightening of the anterior marginal curve alone defines 

 the length of the cardinal curve above it. The posterior 

 cardinal curve is rounded to the hinge, and the lower edge 

 of the valve is broadly rounded. 



Sculpture. (2) The interior shows a large muscle scar near 

 the hinge-line, behind the ocular tubercle ; and also an 

 unusually long straight groove, directed backward and 

 downward, in front of the tubercle ; a fainter, shorter 

 groove directed toward the anterior margin lies in front 

 of this. A thickened band of the shell substance making 

 a slight ridge on the inner surface, exteuds from the post- 

 erior cardinal angle, around the ventral slope of the shell 

 to the anterior marginal curve. The following are mea- 

 surements of some valves; 



Left valve, length 3 mm; width 2\ mm; depth i mm. 

 Eight " 



Carapace " 



As compared with the species of the Protolenus zone — 

 this species is a little larger than B. oculata, from which it 

 is easily distinguished by the character of the surface or- 

 namentation ; in the Etcheminian species the pits are 

 coarser and closer together, and it thus has a rougher sur- 

 face than the species above named. The sculpturing is 

 more like that of Isochilina ventricosa- , which, however, is 

 a much larger species. P. aurora of the Protolenus 

 zone is nearly of the same size, but it differs in the strong 

 anterior furrow, and in its finely pitted surface. 



31- 



. 21 " 



a 



1 



3 " 



" 21 "■ 



a 



n 



2f " 



" 2 " 



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