a general consideration and balancing of all the parts of lichens, 

 their external and internal structure, and as exhibiting their combin- 

 ation with the Algae and Fungi to which Lichens are intermediate. 

 To facilitate the student's labour in the ready and accurate deter- 

 mination of his specimens, the species, especially in the more 

 extensive genera, such as Lecanora, Lecidea, Verrucaria, &c, have 

 been thrown into small groups, distinguished either by the pecu- 

 liar external character of the thallus or the forms and colour and 

 septa of the spores, and in the largest genus, Lecidea, each of 

 such groups is again subdivided into lesser ones according as the 

 hypothecium is dark or colourless. Objectionable as such an 

 arrangement confessedly is in separating allied species, it is believed 

 it will be nevertheless found convenient and useful. In the diag- 

 noses the prominent and distinguishing characters are printed in 

 italics, and the chemical re-action added as a confirmatory aid. 

 Its general habitat, relative frequency or scarcity, date of first 

 discovery in Britain, (also appended afterwards to the name of its 

 discoverer,) are given. The synonymy has been purposely made 

 concise, retaining only a reference to the work in which each lichen 

 was first described and named, and also to the works of Acharius, 

 the Linnseus of Lichenology ; and finally to the various works of 

 Dr. W. Nylander ; and to those of humbler students, where neces- 

 sary. By a reference to these works the synonymy may be, if 

 desired, followed out in extenso. Keferences are made to charac- 

 teristic figures of the lichen, its spores. &c. Micrometric measure- 

 ments of the spores are also given, from the works of Mudd, 

 Nylander, and T. M. Fries, to which are added my own measure- 

 ments. 



Keferences are made to various published Exsiccati, Continental 

 and British, enumerated hereafter ; and in the preparation of the 

 work the Herbaria of Mr. Dawson Turner, Mr. Borrer, and the 

 Hookerian Herbarium at Kew; many original specimens from 

 Acharius ; the collections of Dr. Greville, Dr. George Johnston, 

 Mr. Carroll, Mr. Crombie, Mr. Larbalestier, and many others 

 have been consulted and examined, and where necessary micros- 

 copically. To which must be also added my own immense 

 Lichen-Herbarium (without exaggeration the finest and most 

 extensive private one, both as to British and Tropical lichens, 

 in the kingdom, and destined at my death to be presented to the 

 National Herbarium at Kew) comprised in 400 quarto cases 

 occupying, bookwise, shelves 120 feet in length. The names of 

 Botanists on whose authority those localities rest are appended, 

 and a note of admiration (!) added where a specimen from any 

 locality has been personally examined. Those localities to which 

 no name is appended have been ascertained by the personal 

 research of the author, and for these he alone is responsible. 



The marvellous discoveries of Mr. Larbalestier in the West 

 of Ireland, and the inferior, but very interesting, ones of Mr. 

 Crombie, Dr. Stirton, Mr. McAndrew, Dr. Bainbridge, and 



