SUPPLEMENT.— TECHNICAL TERMS. 



CXCVll 



IV. CONCERNING TECHNICAL TERMS. 



In order to secure, so far as possible, a uniform value for the terms descriptive 

 of the dimensions of whorl and umbilicus, it has been found advisable to use some 

 definite standard of proportion. Such a standard is furnished by the radius— the 

 length from the centre to the periphery. This being taken as 100, the percentage 

 of other dimensions may be approximately stated as follows : 



TPerstenogyral, 

 A Perh 



To 17 per cent A Perleptogyral, 



[Perangustumbilicate. 



From 1 7 per cent. J f i' 



to 34 per cent. | 4 Gp °^ T& ' 



(Angustunibihcate. 



n 04 i. (Substenogyral, 



From 34 per cent. _. . . J aj , 

 *„«n * ~, + Subleptogyral, 



to 50 per cent. 



^ „ n (Subplatygyral, 



From 50 per cent. L , / 

 to 66 per cent, Subpachygyral, 



[ S ubangustumbilicate . 



[Sublatumbilicate. 



From 66 per cent. P 1 ^™ 1 ; 

 4.«qq ™,™.* jPachygyral, 



[Latumbilicate. 

 Terplatygyral, 

 Perpachygyral, 

 Perlatumbilicate. 



to 83 per cent. 



From 83 per cent. I 

 to 100 per cent. | 



When the dimensions exceed 100 per cent, they may be denoted by affixing the 

 word extreme-. Thus certain species might be extremipachygyral, or extremilatum- 

 bilicate, with further modification, when necessary, hy per- and sub-. 



There is a certain arbitrariness about the whole method, as when only a slight 

 difference each side of a dividing line gives a different designation, while more differ- 

 ence, if falling at beginning and end of a division, does not give a different term, 

 although it would be desirable in specific distinction. When a dimension falls on 

 the dividing line, it seems advisable to take the lower denomination as the term. 

 Proportional triangles, such as those given by P. Reynes in the frontispiece of his 

 Monograph, are suitable for taking the measurements. 



The descriptions from page lxv onwards have been checked in accordance with 

 this more exact scheme. 



V. GENERIC CLASSIFICATION. 



The following is a classification of the genera which have been described. 

 They have been arranged in such order in their respective groups that, so far as 

 possible, the sequence shall be from the least to the most modified form ; and the 

 sequence of the groups themselves is on the same basis. 



