BY W. D. FRANCIS. 113 



in my preparations, but the size and shape of the cells 

 and their arrangement in the thallus are closely similar 

 to that described 4 5 6 7 ^nd figured 5^7 under Hilden- 

 brandtia. Both marine and freshwater species are included 

 in the genus which is widelj' distributed in different 

 parts of the world. 



The peculiar distribution of the black coatings on rock 

 surfaces, as outlined above, is exhibited bj' the alga groA^ang 

 on the rocks ; the irregular margins and uncoated patches 

 on coated surfaces are common features of its growth. 

 When seen by me four years ago, the alga was as abundant 

 on rocks in streams of the rain forests (which have since 

 been felled) as the black coating was in felled areas. I 

 have seen the rock}^ bed of a stream coated with, the alga 

 over an area of several square yards. The confinement 

 of the alga to the rain forests on the one hand, and the 

 maximum abundance of the black coatings in the felled 

 areas on the other, suggested that the process of clearing 

 may have accelerated and increased the production of black 

 coatings by killing the alga. The occurrence of the alga 

 on all kinds of rocks in the streams is another feature in 

 which it corresponds with the black coating. Unlike 

 lichens, which are generall}'' dependent upon the substance 

 of the rock the}^ incrust for mineral food material, the 

 alga is sustained b}' dissolved (and suspended) material 

 in the water and is independent in that respect of the rock 

 it incrusts. 



Rock sections showing the thickness of the incrusting 

 alga and the black coating were prepared by a lapidary. 

 Unfortunately, only small portions of the black coating 

 survived the grinding process, but sections of consideraljle 

 extent of the alga remained in position, incidentally 

 showing its extraordinary adherence to the rock surface. 

 To supplement the material for studying the thickness 

 of the black coating, a small piece of the black-coated rock 

 with two plane surfaces, was ground. From these prepara- 

 tions, the following measurements were ascertained : — 

 Alga from about ofi to 81^^ ; black coating from about 

 3/Li to 41u. The black coating is appreciably thinner than 

 the ailga. It appears feasible that a considerable amount 



of reduction in size may take place in the thickness of the 

 H 



