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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 purposes to have a separate name for 

 each variety that can be shewn to be characteristic of a particular geo- 

 logical 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. inequiradiata, S. demissa, 8. concava. 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 the 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 begins to be bent down 

 to form the deflected margin which runs all round the edge and becomes 

 obsolete on approaching the cardinal 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 occa- 

 sionally adult individuals may be found, which on a side view give the out- 

 line of an uniform flattened arch from beak to front. In by far the 

 greater number of 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. rhomboidalis^ which also belongs to this group, the front of 

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



