106 
some puzzling features and it has proved very difficult to 
decide whether they represent two independent varieties 
and correspond to A. fistulosus, Hook, and A. fistulosus, 
Hook, var vermicularis, Griff. recorded by Batters in his 
“List of British Marine Algae’ and described by Harvey 
in “‘ British Marine Algae,” p. 43. 
On the Isle of Man coast the two varieties are distinguish- 
able by their difference in size and by the distribution of 
the sori. One form is attenuated and shghtly acuminate 
and may often be of restricted size. Sterile plants of this 
type are not easy to distinguish at first sight from young 
plants of Scytostpbhon lomentarius. Moreover, the limits 
of individual sori are lost by confluence of the soral margins 
so that the whole plant may be covered with crowded 
sporangia, paraphyses and hairs. Such plants are typical 
winter forms of the species and persist at somewhat high 
levels of the shore and round the margins of deep pools at 
mid-tide zone. They are frequently, though not invariably, 
covered by a well-marked mantle of mucilage hairs, a 
feature which is most conspicuous in early spring. The 
sporangia are almost exclusively plurilocular in character. 
During the summer when conditions are more favourable 
to algal growth, another type of plant agreeing with the 
form regarded as typical of the species is found. These 
plants appear about April; they are of sturdier build than 
their predecessors and gradually invade the pools of the 
upper and middle regions of the littoral zone. They are 
characterised by greater size than the winter plants 
achieve, by well-defined independent sori and by a 
scarcity of mucilage hairs. 
Though these two forms are quite distinct from each 
other, they are connected by a series of intermediate 
stages in which the width of the thallus, the definition of 
the sorus and the copiousness of hair-production are all 
independently variable. : 
