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. 
