76 A NATURALIST ON THE "CHALLENGER." 



With regard to the present vegetation, as stated by Darwin 

 and Boss, there are no aerial plants on the rocks, not even a 

 lichen; I found however a microscopic alga (Protococcus affinis), 

 growing on the guano in. sheltered places and colouring it of a 

 dull green. In the stagnant pools on the rocks grow two low 

 green algae, Prasiola minuta and Oscillaria sordida, and a few 

 diatoms. 



The rocks are poorly supplied with the larger species of 

 seaweeds, apparently because these are unable to endure the 

 constant heavy surf. The high-tide mark is formed by a band 

 of a pinkish white nullipore (Lithothammion polymorphum) ; its 

 calcareous masses form an incrustation on the rocks, in places 

 two inches in thickness, and which is bored in all directions by 

 tubicolous annelids, and has its surface thus pierced all over by 

 small round holes. This band is referred to by M'Cormick as 

 the work of coral insects ; there are no corals at all about the 

 rocks, except in deep water. 



Above the band of Lithothammion is a band of dark red 

 staining on the rocks, caused by an encrusting alga (Hilden- 

 brandtia expansa), and from the region of the tide mark depends 

 a filamentous brown seaweed [Chonospora atlantica). The 

 green weed (Caulerpa claviferd), of which the noddies build their 

 nests, grows in from two to twenty fathoms about the rocks. 



Of the whole of the eleven species of non-microscopic algse 

 belonging to the rocks, two are peculiar, and the remainder are 

 known to occur at widely different localities at the Cape of 

 Good Hope, east coast of Australia, Venezuela, &c* 



I went out for a second night's fishing. The fish for some 



reason did not bite so well as before, having possibly, like the 



birds, profited by experience ; but the men in one of the cutters 



alongside us, kept up a succession of songs with hearty choruses, 



and with the aid of rum and beer and the moonlight, and an 



occasional bite, the time soon passed away until midnight, when 



our boat returned to the ship with a party which had been 



stationed on the rocks to observe stars for determination of 



longitude. 



* Prof, G. Dickie, "Algse collected at St. Paul's Rocks." Liiin. Jour. 

 Botany, Vol.. XIV, p. 311. 



