viii ADVERTISEMENT. 



engaged on the Phycologia Britannica, now in course of 

 publication, that I have not had sufficient leisure to study 

 the fresh-water species with the care that their intricacy 

 demands. 



No great changes have been made in the descriptive 

 portions of the work, with the exception of improved ge- 

 neric and specific characters where such improvement 

 seemed needed ; and the introduction, at the commence- 

 ment of each Order, of short descriptions and remarks in 

 illustration of the variations in habit observed among the 

 species, their geographical range, and anything peculiar 

 connected with their history. 



The general Introduction has been, with some small 

 corrections, for the greater part, retained ; very little has 

 been added, but several passages have been struck out, 

 the substance of which will be found embodied in the in- 

 troductory observations prefixed to the orders. The most 

 important improvement in this new edition consists in the 

 plates to illustrate the genera. These, it is hoped, will be 

 found sufficiently full to enable the student, with the help 

 of the descriptions, to ascertain the genus to which any 

 sea-weed he may find belongs. It is not intended, in a 

 work like the present, to give full analytical or anatomical 

 details in such figures, but sufficient analysis is given for 

 practical purposes. 



Trinity College, Dublin, 

 June 30, 1849. 



