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MISS R. HARRISON AND PROF. S. J. HICKSON ON 



vary is purely arbitrary, but the number of individuals which vaiy 

 about these points is a real number, and represents the two 

 distinct growth-modes of two different species. If the two types 

 were varieties of a single growth-mode, the variation might be 

 expressed by such a diagram as text-fig. 216 B, but that would 

 represent a condition of affairs wholly contrary to the numerical 

 facts ; for the intermediates, which in reality are less than a 

 thirtieth of the whole number, here represent the greater number 

 of forms. 



Text-fiff. 216. 



A. — A diagrammatic representation of two species wliich varj' about two distinct modes, 

 the extremes of which converge towards one another. The abscissae give numbers 

 which vary about each mode respectively. 



B. — A diagrammatic representation of a single species varying about an imaginary mode. 



Type I is undoubtedly the H. cequicostatus of Milne-Edwards 

 & Haime, although, as I shall show, there are considerable 

 variations within the type. Type II is probably identical with 

 Yerrill's [42] H. alternatus, a species which has escaped the 

 notice of several authors. This species possesses all the 

 characters which separate Gardiner's Type II from Type I, — the 

 base smaller than the disk, with a slight constriction above it, and 

 then walls spreading obliquely outwards to the edge of the disk, 



