EXPLANATION OF THE PLATES. 



All the Plates were lithographed by Mr. Huth, Edinburgh, from my pencil draw- 

 ings, with the exception of figures 1 to 5 on Plate X., for which I am indebted to 

 Professor H. N. Moseley. 



The following were the powers of the microscope used by me in making most of 

 the drawings for the Plates (the abbreviations are made use of in the explanations 

 of the figures). 



S. 1 = Swift's 1 inch objective, magnifying about 50 diameters. 



S. i= „ i „ „ 230 „ 



S. i= „ i „ „ 330 „ 



Z. yW = Zeiss' yW objective (oil immersion), magnifying about 950 diameters. 



LIST OF THE ABBREVIATIONS. 



