A very common parasite on Cystoseir a fibrosa, whose branches 

 are rarely found free from the olive-coloured soft pencils of this 

 little plant. In size and appearance to the naked eye there is 

 much resemblance to Elachistea fucicola (Tab. CCXL.), except 

 that the colour is generally greener, and the length of the 

 tufts rather less ; but under the microscope these species are 

 very readily known from one another. E. fiaccida is remark- 

 able for the shortness of its articulations, in proportion to their 

 breadth throughout the lower and middle portions of the fila- 

 ments, and for the gradually increasing length of the cells towards 

 the apices. The filaments, also, taper exceedingly at the base ; 

 and the tubercle from which they originate is of very much 

 smaller size than in & fucicola. 



Fig. 1. Tufts of Elachistea flaccid a growing on Cystoseira fibrosa. 2. 

 Vertical section of part of a frond, showing a portion of the tubercle, with 

 paranemata and spores, and part of two filaments. 3. Apex of a filament. 

 4. Spore, with its paranemata : — all magnified. 



