EXPLANATION OF PLATES. 
All the ligures are reduced to about one half of their original size. The 
most of them have been drawn with a camera lucida, and with various combi¬ 
nations of lenses; the water immersions Zeiss J and K, and the oil immersions, 
Zeiss 2mm. and Leitz and together with oculars Zeiss 12 and 18 were 
principally used. The magnification given with each figure is the original 
magnification of the drawing before reduction. 
