52 Silurian Fossils of Canada* 



begins to be bent down to form the deflected margin which rims 

 all round the edge and becomes obsolete on approaching the car- 

 dinal angles. This margin varies in width from one-twelfth to 

 two-thirds the whole length of the shell and therefore the disc 

 in some of the varieties occupies nearly the whole superficies of 

 the valve, but in others, less than half. In the very young shells 

 in most of the specimens that I have seen, there is no deflected 

 margin and occasionally adult individuals maybe found, which on 

 a side view give the outline of an uniform flattened arch from 

 beak to front. In by far the greater number of the specimens 

 however the deflected margin is well defined. The contour of 

 the front of the visceral disc varies according to the form of the 

 deflected margin and is thus either broadly rounded or more or 

 less pointed. In S. depressa, which also belongs to this group, 

 the front of the disc, and its sides also, are often nearly straight. 



The dorsal valve is flat or only gently concave beneath the 

 visceral disc of the ventral valve, but all round, its curvature con- 

 forms to that of the deflected margin. 



In the t'^ue S. alternata the areas of the ventral and dorsal 

 valves are inclined towards each o her at an angle varying from 

 75° to 80°, but this angle never amounts to 90°. It will be ob- 

 served 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 

 strise, 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 following descriptions. The radiating striae are also very 

 constant in the aspect they present. The only parts which appear 

 to afford permanent variations of much value are the front of 

 the visceral disc the deflected margin and the hinge line. 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 characters. 



