Cambrian Rocks in Cape Breton. 455 



The following are measurements of some valves : — 

 right valve, length 3| mm., width '2\ mm., depth | mm. 



(C ££ 



" H 





2i 





" 1 '< 



(( C( 



O 





2 





" 1 ' 



(( (C 



" 3 





2i 





" 1 ' 



li 11 



" 31 





21 





" 1 ' 



carapace 



" 3 



Mutations. 



2i 





« 2 ' 



In the. lower Etcheminian Fauna some forms occur 

 which may be referred to this species as mutations. 



Assise E. 1. h. mut. ohesa, n mut. 



A broader and more tumid form than the type. The 

 hinge-line is three-fifths of the length of the valve and 

 there is a tubercle at the posterior end of the hinge-line 

 The ocular tubercle is prominent and enclosed by the fur- 

 row ; a row of low and obscure tubercles runs curving 

 from the ocular tubercle to the lower angle of the valve 



SculjJture. The surface is marked by a fine punctation, 

 and by anastomosing 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 3mm, width 2^mm, depth two valves to- 

 gether l|mm. 



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

 the preceeding and smaller. 



This form differs from the young of Bradorona observator 

 of the lower fauna in the deeper furrow around the tuber- 

 cle, and in the rounder base of the valve ; hence we have 

 associated it with B. vigilans 



Assise E. 1. d. An imperfectly preserved right valve 

 was obtained here from beds of feldspathic sandy shale. 

 It is considerably smaller than the type, and the surface 

 is rough and dull from corrosion. 



