46 



Dorsal valve strongly elevated, narrowly rounded or subangular along 

 the median line, thence descending with a somewhat concave slope to the 

 sides. (Fig. 3a). The mesial fold is very obscurely indicated in large speci- 

 mens. Along the crest of the median line there is a single laj-ge rounded 

 rib, about twice the size of the others ; below this, on each side, there 

 are four ribs which may be said to belong to the fold, as thfere is between 

 the fourth rib and the fifth a shght depression which may be seen, in 

 some specimens, running from the front margin upwards ; from this the 

 slope is at first nearly flat or very slightly convex, becoming concave on 

 approaching the cardinal angles, and more convex towards the margin 

 in the anterior half ; the umbo is obscurely indicated ; the beak scarcely 

 distinct from the cardinal edge ; the area is about one line high at the 

 beak, and slightly overhangs the hinge-line. 



This shell is so abruptly truncated, that when placed on the front mar- 

 gin it stands upright. On a side view, in this position the suture between 

 the two valves is seen to be strongly curved towards the ventral valve ;. 

 the median line of the dorsal valve rises almost vertically, at a right 

 angle, to the truncated front margin. The lower two-thirds is gently con. 

 vex. In the upper third, it is abruptly curved down to the beak, which 

 it reaches almost at a right angle to the vertical plane. (Fig. 35). The size 

 of the ribs is somewhat variable. In specimens two inches wide there are 

 from three to four ribs in the width of three lines, at the front margin 

 Along the median line of the dorsal valve, forming the crest of the mesial 

 fold, there is a single large rounded rib, two lines wide ; in the bottom 

 of the mesial sinus of the ventral valve there is also a large rib, but con- 

 cave ; the cardinal angles and the umbones of the two valves are nearly 

 smooth ; the ribs vary from moderately convex to nearly flat ; those on 

 each side of the mesial fold appear to be bifurcated near the beak, all the 

 others undivided. The surface is also more or less marked by concentric 

 wrinkles of growth, and there appears also to be a set of fine concentric 

 striae. 



This species differs from S. arenosa, in having a much smaller area, a 

 much deeper mesial sinus in the ventral valve, a larger fold on the dorsal, 

 and a large undivided rib along the mesial line of both valve?. 



Small specimens, about an inch wide, have the upper half of the sides 

 straight and often at right angles to the hinge line. 



Locality and Formation, — Indian Cove, Gaspe limestone, No; 8. 



Colhdo 's— Sir W. E. Logan, R; Bell. 



