Vlll 



YORKSHIRE TYPE AMMONITES— II May 



An innovation will be made in this volume — an extensive use of 

 the Measurement Table, published in the Appendix of Vol. I (facing 

 p. d) ; and it is necessary to explain this before proceeding further. The 

 proportions of the types will be given in the following manner, for example : 

 30, 22, 22, 55. These figures stand for — Diameter, 30 mm. ; Breadth of 

 whorl, 22 per cent, of the diameter; Thickness (maximum) of whorl, 

 22 per cent ; Umbilicus (width), 55 per cent. As the figures will always 

 be given in the same order in a formula, it will be quite unnecessary to 

 repeat the details, the sequence will always read Diameter, Breadth, 

 Thickness, Umbilicus. In the case of the Comparable Species, the 

 proportions will be added after the reference, so far as circumstances 

 permit of their being ascertained. And if any one proportion cannot be 



given a dash will be placed, thus, 30, 22, — , 55, so as to retain the 



same stereotyped formula. 



These formulae will be preceded by certain letters, namely, 



S = specimen, T = text, F = figure, to indicate which have been used ; 



for sometimes delineations are incorrect, and sometimes the figures in 



the text do not agree with those ascertained from the delineations, 



through misprints, incorrect drawing, or miscalculation. 



Figures in brackets, after the first proportion, thus 33 (63), indicate 



the actual size of an enlarged (or reduced) delineation, the first figures 



indicating the actual size of the specimen. 



To return now to the Terms : it is advisable to make use of only 

 the one set of figures. Therefore, in order to meet the case, it is necessary 

 to call the radius 50 per cent, of the diameter. Then the six series of 

 percentages can be divided among the 50 per cent, of the diameter taken 

 as the general base-line instead of the radius. This will enable the 

 terms to be employed in the following manner : — 



To 8% C Perstenogyral 



of diameter J Perleptogyral 



\Perangustumbilicate 



From 8 fStenogyral 



to 17% J Leptogyral 



[^Angustumbilicate 



From 17 f Substenogyral 



to 25% J Subleptogyral 



\Subangmtumbilicate 



From 25 C Subpiatygyral 



to 33% < Subpachygyral 

 \Sublatumbilicate 



From 33 CPlatygvral 



to 42% J Pachygyral 



\Latumbilicate 



From 42 CPerplatygyral 



to 50% < Perpachygyral 

 I Perlatumbilicate 



