125 



Strophomena Thalia. N. s. 



SsROPHOMENA Thalia. — Can. Nat. and GeoL, Vol. V., p. 5Y, Feb. 1860. 

 a b 



Fig. 103. 

 Fig. 103. — Strophomena Thalia, a. longitudinal section ; b, ventral aspect. 



Description. — Semi-oval or sub triangular, often narrowly rounded or 

 somewhat pointed in front, hinge-line usually greatly exceeding the width 

 of the shell, and forming with the sides an angle of from 70° to 80°. 

 Width at hinge-line from one to two inches, length about five eighths the 

 width. 



Dorsal valve moderately convex, depressed towards the cardinal angles, 

 which are a little recurved ; umbo flat. On a side view the outline forms 

 a gentle and nearly uniform curve from the front for about four fifths the 

 length, when it descends with a flat slope to the beak, which it reaches at 

 an angle of from 45° to 60°. 



Ventral valve concave, the greatest depth about the middle or a little 

 nearer the beak. 



Area of ventral valve moderate, forming an angle of about 100° with 

 the plane of the margin, its height in a specimen two inches wide, one line ; 

 foramen triangular, closed by a convex deltidium, its width at the base 

 about one fifth greater than the height. The beak is not perforated in 

 any specimen that I have seen. Area of dorsal valve nearly in the plane 

 of the margin, its width about one third of that of the ventral valve. 



Surface with moderately coarse radiating striae, which increase both by 

 bifurcation and interstitial addition, usually unequal but sometimes uniform 

 in size, from ten to fifteen in the width of two lines, crossed by excessively 

 fine crowded concentric lines. 



This species is aUied to S. fluctuo.m, but differs in having the areas of 

 the ventral and dorsal valves inclined at an angle which is rather less than 

 a right angle. In its outline it forms nearly an uniform arch instead of 

 being abruptly bent like jS. fluctuosa. 



