120 



intermediate degrees of convexity, and some in wHcli the deflected margin 

 can be detected with a width equal to half the whole length of the shell. 



The surface is the same as that of S. alternata. 



The area of the ventral valve is one line high in a specimen fourteen 

 lines wide, and hes very nearly in the plane of the lateral margin. The 

 foramen is as wide as high, and closed by a strongly convex deltidium, 

 the lower margin of which is concave to admit the equally convex 

 deltidium of the dorsal valve, whose area is almost half a line wide and 

 forms an obtuse angle of between 90° and 100° with that of the ventral 

 valve. The beak of the ventral valve exhibits in some specunens a small 

 round perforation. 



This species differs from S. nitens in being in general a little longer 

 proportionally, larger, and more uniformly convex, with scarcely a distinct 

 deflected margin. In *S'. nitens the length is in general only two thirds of 

 the width, but in this species it is always over five sixths. 



The angle formed by the inclination of the areas being obtuse instead 

 of acute furnishes the only character as far as I can ascertain by which it 

 can be separated from S. alternata. 



Locality and Formation. — Charleton Point, Hudson River group, and 

 also at East Point in the Middle SUurian, Anticosti. 

 Collector. — J. Eichardson. 



Steophomena Leda. (Eillings.) 



Strophomena Leda. — Canadian Naturalist and Geologist, vol. 5, p. 55, Feb. 1860. 



Fig. 98. Pig. 99. 



Pig. 98. — Strophomena Leda with a portion of the hinge area of the rentral 

 valve enlarged to shew the striated teeth. 

 99. — A specimen without ears supposed to be of the same species. 



Description. — Shell rather small and thin, semi-oval, the front and front 

 angles regularly rounded, sometimes a little narrower at the base of the 

 ears than at one third the length from the hinge line, the latter usually 

 exceeding the greatest width of the shell, and forming projecting spiniform 

 ears. Width excluding the ears, five to nine lines ; length five-sixths of 

 the width ; ears one line and a half in length each, in a well preserved speci- 

 men five lines wide. 



