144 



slightly concave in the upper third ; very gently convex in the middle and 

 broadly below into the ventral margin. The latter is gently and uniformly 

 oonvex in the middle, and broadly rounded upwards at each end. The 

 posterior extremity is nearly at a right angle to the hinge-line in the 

 upper half, straight or slightly concave. In the lower half it is obtusely 

 rounded into the ventral margin. In one specimen, the lower half projects 

 two or three lines further back than the end of the hinge-line. 



The right valve is moderately convex, depressed, concave, flat, or very 

 slightly concave, below the posterior half of the hinge. 



One specimen shows the impressions of three lateral teeth close beneath 

 the hinge in the middle third. Another exhibits four small anterior teeth. 



Surface concentrically striated, four or five in two lines, closer and sub- 

 lamellose on the small anterior wing. 



Length of an average specimen, twenty-one lines ; height, eighteen. 



Locality and Formation. — Arisaig, Nova Scotia. Upper Silurian. 

 Collector. — T. C. Weston. 



