vi PREFACE TO THE FOURTH EDITION. 



I am also indebted to Dr. Traquair for allowing me to 

 figure some of the specimens in the Edinburgh Museum 

 of Science and Art. In regard to the illustrations, I 

 may further say that in almost every case they have 

 either been derived from original memoirs and works of 

 reference, or drawn from specimens. 



J. A. T. 



THE UNIVERSITY, 



ABERDEEN, April 1906. 



