ceramiacejE. 223 



species are distinguished by microscopic characters, and 

 several of them rest on very slight and uncertain foun- 

 dations. They are, moreover, very variable, and are 

 altogether more diflScult than those of perhaps any other 

 genus to determine with certainty. The rosy colour, 

 feather-like form, and delicate texture of the fronds 

 of most of the species cause them to be conspicuous 

 for beauty, even in an assemblage which includes such 

 potent rivals as are to be found among the Red series 

 of British sea-weeds. 



Section 1. — Frond shrub -like ; the stem and Iranches veined, 

 indistinctly jointed ; branches pinnate ; branchlets alternate, 

 or on one side only of the brcmch. 



Callithamnion arbuscula. The bush Callitham- 

 nion. 



Fronds growing several from the same base, from two to 

 six inches high ; stems naked below, without visible joints ; 

 branches alternate, those of the second series thickly set 

 with minute plumules, which are densely crowded towards 

 their tips ; plumules furnished with alternate, simple or 

 forked, spreading, recurved pinnules. Pavellse in pairs, 

 springing from the stem of the plumules ; tetraspores 

 spherical, stalkless, plentifully produced on the upper edge 

 of the pinnules of the plumules. 



This is one of the most robust of British Callithamnia, 

 and thrives in the roughest water, and on the most ex- 

 posed rocks. It grows abundantly on the west coasts of 

 Scotland and Ireland, and is rare in England. It is 

 perennial, and fruits in summer and autumn. 



