446 Canadian Record of Science. 



more rounded at the two ends and below, than the type 

 and is flatter, but is of the same 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 fur- 

 row is visible along the anterior marginal curve ; an ob- 

 scure row of tubercles extends along and near the anterior 

 half of the hinge of line. 



Scidpture. Surface pitted, the spaces between the punc- 

 tures becoming anastomosing ridges near the margin, and 

 jDresenting ridges on the posterior lialf of the valve sub- 

 parallel to the margin. 



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



Horizon and locality. Assise E. i c. Lower Etcheminian, 

 at Dugald Brook, Scarce. 



Mutation magna, n. mut. PL I., figs. 11 a and h. 



Valves rather tumid, hinge-line shorter than in the 

 type, two fifths of the length of the valve ; ocular tubercle 

 prominent, behind, and around it is a shallow furrow ; car- 

 dinal curves of the margin long. No marginal furrow is 

 visib I 



Scatqjture. Surface pitted ; there are obscure anastomo- 

 sing ridges between the pits parallel to and near the mar- 

 gin ; a narrow ' obscure band extends from the ocular tu- 

 bercle obliquely backward and downward. 



Size. Length, 5 mm ; width, 4 mm. ; depth, of a valve 

 Ih mm. 



Horizon and locality. Assise E. 2 b. Lower Etcheminian 

 at Dugald Brook. Scarce. 



Mutation major, n. mut. PL 1. figs. 10 a and b. 

 Valves rather tumid, hinge-line about three fifths of the 

 length of the valve. Cardinal curves rounded ; no mar- 



