Sihirian Fossils of Canada. 59 



mesial fold extended into a linguiform projection of the middle 

 of the front margin gives to the dorsal valve a trilobate character. 

 The area of the ventral valve is in general at right angles to the 

 plane of the lateral margins, but sometimes it slopes a little for- 

 ward. In some the hinge-line is greatly extended, the cardinal 

 extremities forming projecting triangular ears. 



Width on hinge-line from one inch to one inch and a half- 

 Length variable, from two thirds of the width to four fifths or a 

 little more. 



Stro'phomena deltoidea (Conrad) has the ventral valve convex 

 and may be always distinguished from this even when the hinge" 

 line cannot be seen by the small rounded umbo close to the beak. 

 >S'. camerata and S. tenuistriata (Conrad) have also the ventral 

 valve convex. (See Plate 31 A, Vol. I. Pal. N. Y.) 



Locality and Formation. — ^Trenton limestone, City of Ottawa, 

 rare ; more common in the Hudson River group, Anticosti. 



Collectors. — E. Billings, F. Richardson. 



Strophomena Thalia. N. s. 



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 YO^ to 80°. Width at hinge-line from one to two 

 inches, length about five eighths the width. 



Dorsal valve moderately convex, depressed towards the cardi- 

 nal 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 459 

 to 60O. 



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 in- 

 crease both by bifurcation and interstitial addition, usually un- 



