165 



An additional example of this species from the bed in which the 

 original was found, and three others from a layer about a foot lower in 

 the measures, give additional information of the species. 



These examples are wider than the type. The ocular tubercle in this 

 species is a little off from the hinge-line. The cardinal slopes are long 

 and the anterior marginal slope is somewhat straightened. 



The following are measurements of some valves : 



right valve, length 3| mm., width 2 mm., depth f mm. Dimensions. 



" " " 3 " 2i " " 1 " 



" " " 3 "2 " " 1 " 



" " " 3 " %i n i 



" " " 3J? " " 2| " " 1 " 



carapace " 3 " " 2 " " 2 " 



MUTATIONS. 



In the lower Etcheminian Fauna some forms occur which may be 

 referred to this species as mutations. 



Assise E. 1 b. mut. OBESA. 



Can. Rec. Sci. Montreal, 1902, p. 455. 



A broader and more tumid form than the type. The hinge-line is Mutations of 

 three-fifths of the length of the valve and there is a tubercle at the pos- this 8 P ecies> 

 terior end of the hinge line. The ocular tubercle is prominent and 

 enclosed by the furrow ; a row of low and obscure tubercles runs curving 

 from the ocular tubercle to the lower angle of the valve. 



Sculpture. The surface is marked by a fine punctuation, and by anas- 

 tomosing ridges on the posterior half of the valve ; a diagonal band of 

 these ridges runs from the ocular tubercle, diagonally backward and 

 downward to the lower part of the anterior margin. 



Size. Length 3 mm., width 2^ mm., depth two valves together 1| mm 

 or more. 



Assise E 1. c. A form occurs here which is flatter than the preceding 

 and smaller. 



This form differs from the young of Bradorona observator of the lower 

 fauna in the deeper furrow around the tubercle, and in the rounder base 

 of the valve ; henfce we have associated it with B. vigilans. 



