6 PREFACE. 



have included comparatively few references to articles 

 which I have not myself consulted either in the original 

 or in abstract. I hope, however, that the more impor- 

 tant papers are included. 



My thanks are due to Mr. A. N. Leathern for the 

 loan of specimens, and to Professor E. A. Minchin for 

 the very kind loan of books from his library and of 

 specimens in his possession, as well as for permission 

 to use his laboratory for my own small researches. 



LONDON, ENGLAND, W. C. B. 



May, 1911. 



