159 



thread-like ridge extending diagonally backward from the tubercle ; this 

 corresponds to a furrow on the inside of the valve. 



Size. Length and breadth each 3 mm. ; depth of a valve 1 mm. 



Horizon and locality. In the dark, brownish gray sandy shale of Assise 

 E. 1 d, Lower Etcheminian, at Boundary brook, Escasonie, Rather 

 common. Also in Assises E. 1 b. and E. 1. d. at Dugald brook ; infre- 

 quent. 



Variety ACUTA. 



Can. Rec. Sci., Montreal 1902, Vol. VIII., p. 447. 



This is a large form, more pointed below than the type. Anterior mar- Variety acuta. 

 ginal slope somewhat straightened. Ocular tubercle distinct. 



Sculpture. Surface minutely punctuate, and showing a strong striation 

 near the hinge. 



Size. Length and breadth each about 4 mm. ; depth of a valve 1 mm. 



Horizon and locality. Assise E. 1 b., Lower Etcheminian, at Dugald 

 brook ; infrequent. 



A small example, supposed to be the young of this form is narrower, 

 more acutely pointed below, and with straighter anterior and posterior 

 marginal curve, was found in Assise E. 1 d. at Boundary brook. 



Mutation SPINOSA. PI. XII., figs. 13 a and b. 



Can. Rec. Sci., Montreal, 1902, Vol. VIII., p. 448. 



Wide below the cardinal curves. Anterior marginal curve straight- Mutation 

 ened. A sharp marginal furrow all around except at the hinge. 



The ocular tubercle is distinct and there are spines at the ends of the 

 cardinal curves, except at the lower end of the posterior curve. 



Sculpture. The surface is minutely punctuate ; on the posterior slope 

 of the valves and on a band descending backward from the ocular 

 tubercle, the pits merge into interrupted strise, divided by inosculating 

 ridges. 



Size. Length and breadth each about 4 mm. ; depth of a valve 

 1^ mm. 



Horizon and locality. An entire carapace in the Assise E. 1 e., Lower 

 Etcheminian, at Dugald Brook ; scarce. 



