PREFACE TO THE FIRST EDITION. ix 



As we are convinced that to any one engaged in practical 

 study, photographs and photomicrographs supply the most 

 useful and exact information, we have used these almost exclu- 

 sively in illustration of the systematic description. These have 

 been executed in the Pathological Laboratory of the University 

 of Edinburgh by Mr. Richard Muir. The line drawings were 

 prepared for us by Mr. Alfred Robinson, of the University 

 Museum, Oxford. 



To the volume is appended a short Bibliography, which, 

 while having no pretension to completeness, will, we hope, be 

 of use in putting those who desire further information on the 

 track of the principal papers which have been published on 

 each of the subjects considered. 



JUNE, 1897. 



