119 
It is possible that these minute cells escaped observation 
in Harvey’s material as they are difficult to see even under 
high magnification and with good illumination. In the 
absence of reproductive structures on Harvey’s Lithocystis 
it is impossible to decide whether it is identical with 
Melobesia mnutula or not. Neither Melobesia minutula 
nor Lithocystis Allmanm appear in Batters’ list of 1902. 
Melobesia minutula would therefore appear to be a new 
record for the British algal flora. 
THE ANALYTICAL KEY 
In compiling the analytical key the authors have been 
obliged to relinquish, though with reluctance, the original 
intention of including the species, and have had to restrict 
themselves to the genera. This curtailment was made 
necessary by the fact that there still exists considerable 
uncertainty about the integrity of many of the species in 
some of the larger genera. A series of critical studies of 
such genera as, for example, Callithamnion, Ceramium or 
Cladophora must be undertaken before any reliable or 
useful specific key could be established. In the genera 
cited, species claimed as distinct are linked by so many 
intergrades that it is difficult to know where one ends and 
the other begins. For example, Callithammion Hookers 
comprises a variety of forms which grade insensibly into a 
condition recognisable as Callithamnion polyspermum. 
See figures 69, 70. 
In constructing the key, keeping in mind the student- 
public for whose service the memoir is planned, morpho- 
logical features or salient characters that attract attention 
have been given first place in diagnostic value. Such 
characters alone, however, are not in all cases sufficiently 
discriminative. As a second line of approach therefore, 
anatomical features have been laid under contribution, 
