PREFACE TO THE THIRD EDITION. 



IN the present Edition I have made several altera- 

 tions in the illustrations and in the text. Figures 

 4, 43, 78, 129, 132, and 160 have been re-drawn, 

 so as to represent more satisfactorily the structures 

 intended to be delineated. In the text a good many 

 verbal corrections have been introduced, and for 

 many of these my thanks are due to my friend, 



Dr. Pye-Smith. 



E. KLEIN. 



January, 1884. 



