118 



The dorsal valve is flat or only gently concave beneath the visceral disc 

 of the ventral valve, but all round, its curvature conforms to that of the 

 deflected margin. 



In the true S. alternata the areas of the ventral and dorsal valves are 

 inclined towards each other at an angle varying from 75° to 80°, but this 

 angle never amounts to 90°. It will be observed that in some of the new 

 species hereinafter described it is greater than 90°. 



The surface in most of the species exhibits two sets of radiating striae, 

 the larger of which are about one-twelfth of a line wide in large specimens, 

 and the smaller half that size, from one to ten of the smaller between 

 each two of the larger, the more common numbers being from three to 

 five. Sometimes also the shell is marked with a series of concentric 

 wrinkles. 



Of the above characters, those which are confined to the upper half of 

 the shell such as the form of the beak, the umbo, the concave depressions 

 or hollows on each side of the umbo, and the reflected cardinal extremities, 

 are common to all the species, and in order to avoid repetition will not be 

 particularly dwelt upon in the foUowing descriptions. The radiating striae 

 are also very constant in the aspect they present. The only parts which 

 appear to aflbrd permanent variations of much value are the front of the 

 visceral disc, the deflected margin and the hinge Hne, The proportional 

 length and breadth of the shell seems also to be of much importance 

 especially if accompanied by a variation in two or three of the other char 

 racters. 



Strophomena nitbns. (Billings.) 



Strophomena nitens. — Canadian Naturalist and Geologist, vol. 5, p. 53, Feb. 1860. 



Fig. 9T. 



Fig. 77. — Strophomena nitens. a, is a section showing the curvature and obtuse angles 

 formed by the inclination of the areas. 



Description. — Transversley semi-oval, sides somewhat straight for one 

 third or a little more of the length from the cardinal angles, and slightly 

 converging towards each other ; front angles broadly rounded ; front mar- 

 gin gently convex or nearly straight for about one third the width in the 

 middle portion. Width on hinge-line from nine to twelve lines. Length 

 from six to eight lines. 



