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INTRODUCTION. 



3. Variational Formula. The range of variation within the limits 

 of a certain specific group may be conveniently shown by formulae. 

 Pour main characters are used in the diagnosis of these groups of 

 specimens: 1. The elevation of the peristome or of the free 

 portion of the zooscial tube (p). 2. The shape of the zocecia (c}. 

 3. The length of the zooscia (I). 4. The nature of the 

 zoarium (r). 



In compiling the formulae, only normal equivalent zocecia can be 

 used. Each letter of the general formula, p, e, I, r, is replaced by 

 a figure to show the extent of the development of the character 

 represented. Thus, if the aperture of the zooecion be flush with 

 the surface of the zoarium, p is replaced by ; if the peristome be 

 slightly raised, it is shown by the substitution for p of 1, if well 

 raised of 2, if highly raised of 3. 



Thus, in the genus Stomatopora the terms of the formula are as 

 follows : 



Intermediate variations may be indicated by the use of dashes 

 beside the figures. 



Thus, we may at once recognize the differences between three 

 species of the genus Stomatopora by comparing their formulae : 



p. c. I. r. 



S. dichotoma (Lamx.) = 2011 



S. dichotomoides (D'Orb.) = 1 2 1 



S. waltoni, Haime = 1020 



The formulae also readily enable us to compare the species of one 

 age with those most closely allied to them which lived in other 



