(160) 



This species can scarcely be confounded with any other ; its densely 

 bushy habit and dark coloiu* are so unlike any other species of the 

 genus, that there can be no great difficulty in identifying it, and once 

 seen, its characteristic features cannot well be mistaken. It has some- 

 what the appearance of C. spongiosum, but the dichotomous, not pin- 

 nated ramuli of that species, will readily identify it. 



G. arbiiscula is said to be rare in England, but not uncommon on the 

 shores of both Scotland and Ireland. The only English specimens we 

 have seen are from Yorkshire, where we believe it is scarce. On the 

 rocky shores of the east of Scotland it is rather plentiful, and the fruit is 

 not scarce, tetraspores being rather more plentiful than favellse. 



Its favom-ite habitat is on exposed rocks which are dry at low water^ 

 seldom growing on the rocks themselves, but on the small live mussel- 

 shells that often form a dense covering in such localities ; in some 

 places preferring the face of the rock fronting the land, and thus as 

 it were seeking protection from the surf which comes rolling in from 

 without ; at other times forming rough, spongy-looking tufts among 

 Foli/sijihonia urceolata, on slojjing rocks exposed to the full force of the 

 waves, almost always growing near the summit of the rock, thus as 

 it were appearing to prefer shallow water ; yet we have never found it 

 gi'owing near high-water mark, and very rarely in pools where it would 

 always be covered with water. 



EXPLANATION OF PLATE CXIX. 



Fig. 1. — CaUithamnmn m-buscula, natural size. 

 2. — Branclilet with favelte. 

 S. — Branchlet with tetraspores. 

 4. — Transverse section of stem. 

 5. — Portion of stem. All magnified. 



