VI PREFACE. 



observed this varietal difference to be very clearly marked 

 between many species common to the north and south of 

 England respectively, and the differences are in many cases 

 yet more strongly emphasized in forms of the same species 

 from different countries. To the expert, these modifications 

 rarely cause embarrassment, but with the beginner the case 

 is very different, and trivial modifications that do not in 

 any way affect the true specific character, are not unfrequently 

 considered as indicating a distinct species. 



It is one thing to recognise a species by some unimportant 

 mark that may be constant in one locality, and another to 

 thoroughly grasp the true specific characters that remain 

 constant in every locality. With the object of lessening the 

 difficulties indicated above, in addition to the specific diag- 

 nosis, extracts have been given from one or more authorities, 

 describing minor variations of colour, texture, form, &c., in 

 the case of species prone to variation. The various works of 

 the late Professor Elias Fries of Upsala, have served as the 

 basis from whence specific characters have been drawn up, 

 and in cases where the description covers the typical British 

 form, have been given intact. It is hoped that all sources of 

 information have been acknowledged. 



As all the individuals of a given species are not cast in the 

 same mould, it will be understood that the various measure- 

 ments given apply to the average size of the part indicated. 



G. MASSES. 

 Kew, Surrey, 1892. 



