Silurian Fossils of Canada* 51 



be in reality a distinct species of itself or only a variety of some 

 other species. Its value to liim as a guide, while tracing out 

 the geographical distribution of these Rocks, is not at all affected 

 by the zoological question. In demonstrating the physical 

 structure of the country, he can reason upon varieties with as 

 much safety as upon true species, provided that such varieties 

 are confined to and consequently characteristic of particular por- 

 tions of the geological edifice. It is therefore, of great impor- 

 tance, not only to ascertain to what particular level each variety 

 is confined, but also to determine whether or not such species as 

 range through several formations exhibit any and what change 

 in form on passing from one group of rocks to another. Should 

 it be hereafter, proved that the supposed species above quoted, 

 constitute on purely zoological grounds, but one extensive and 

 variable species still it would be convenient for geological pur- 

 poses to have a separate name for each variety that can be shewn 

 to be characteristic of a particular geological horizon. 



Before entering upon the description of the new species, I shall 

 give a general account of such characters as are common to all 

 the forms of the group typified by S. alternata, 



Strophomena alternata, (Conrad.) 



In all the forms of this important type, the convexity of the- 

 ventral valve has a peculiar contour which may be seen not 

 only in the Lower Silurian but also in the Upper Silurian and 

 even in such Devonian species as S. Pattersoni, S. inequii-adiataj 

 S. demissa, S. concava and others. That part of the valve which 

 is usually called the visceral disc occupies all the central region 

 of the shell and terminates in a point at the beak. Just in front 

 of the beak it forms a more or less well defined low rounded 

 umbo on each side of which there is a flattened or sub-concave 

 depression extending obliquely outwards to the margin just in 

 front of the cardinal angles. These latter are usually reflected 

 or a little curved upwards from the plane of the lateral margins. 

 The visceral disc is somewhat flattened, gently convex or only 

 slightly elevated throughout the greater part of its extent. In 

 tbe upper half of the shell it is bounded by the depressions that 

 have been mentioned as existing between the umbo and cardinal 

 angles, but in front and at the sides it terminates where the shell 



