vi PREFACE 



hundred terms when ill-health prevented his continuing the 

 work, and it was handed over to the present authors. 



We have to acknowledge our indebtedness to Professor 

 J. H. Ashworth of Edinburgh University, who has very kindly 

 read over the manuscript and given us some suggestions for 

 additions. 



I. F. H. 

 ZOOLOGY DEPARTMENT, 



UNIVERSITY OF BRISTOL. 



