EXPLANATORY NOTE. 



On account of the length of the following paper, it has 

 been found necessary to publish it in two parts. All the 

 plates will appear at the end of the second part in Volume 

 XXXIX, and to prevent confusion with plates of other 

 papers in Volume XXXVIII, only the figure numbers (which 

 are consecutive), are given in the text of the first part. 



