VI PREFACE. 



Owing to war-time restrictions it has not been 

 possible to obtain Dr. Penard's assistance, but my 

 thanks are due to him for his permission to reproduce 

 several of his published figures. I am also indebted 

 to Mr. J. M. Brown for placing his records and 

 descriptions at my disposal, and to Mr. Hopkinson 

 for undertaking the bibliographical work and for his 

 assistance in seeing the volume through the press. 



G. H. WAILES. 



SHEFFIELD, 



25th November, 1918. 



