Note on a Hydrocoralline from Rockall. 61 



V". — Note on a Hydrocoralline from Rockall. 

 By Professor J. Arthur Thomson, M.A. 



(Read 20th December 1909. Received 20th December 1909.) 

 I received this summer, from a trawler, three pieces of an interesting 

 Hydrocoralline brought up near Rockall, — that lonely granite rock in the 

 North Atlantic (57° 36' N. lat.; 13° 41' W. long.),— 184 miles west of 

 St Kilda, 260 from the North of Ireland, 290 from the nearest part of the 

 mainland of Scotland. As northern records of Hydrocorallinse are few and 

 far between, it is of interest to register this one. 



The specimens are white flabellate colonies which agree with the description 

 and figures of Stylaster gemmascens (Esper) given by Professor P. Martin Duncan 

 {Trans. Zool. Soc, viii., 1874, p. 332, pi. ix. 12 figs.). The diagnosis, quoted by 

 Duncan from Milne-Edwards and Haime, reads : — "The corallum is subflabelli- 

 form. The branches often coalesce, and the younger are crowded with small 

 granulations, which are irregularly placed between the calyces. The old branches 

 are almost smooth. The calyces are alternate on young branches, and sparingly 

 developed on the old ; they are circular, oval, or deformed, and have projecting 

 margins. There are from twelve to sixteen septa, which are often irregular." 



The surface of the colony shows the " cups " that are characteristic of 

 Stylasterids. They occur all round the smaller branches, but are chiefly 

 lateral on the larger. Each " cup " consists, as is well known, of the cavity 

 of a nutritive polyp or gastrozooid, surrounded by a circle of twelve or so 

 smaller cavities lodging the tactile dactylozooids. Each cup bears a 

 deceptive resemblance to the calyx of a Madrepore, a resemblance heightened 

 in some cases, notably in Aulopora, by septa-like ridges extending inwards 

 from the dactylozooids. It is historically interesting to notice that in Martin 

 Duncan's memoir of 1874, where this form is beautifully figured, it is still 

 misinterpreted as a Madreporarian, with which it has, of course, nothing- 

 whatever to do. To get a general picture of the nature of a Hydrocoralline 

 colony, we have to imagine a much-branched hydrorhiza in which lime is 

 secreted from the tubes instead of a perisarc, so that numerous fine canals 

 are enclosed in a coherent calcareous framework. To this we have to add 

 that the polyps are dimorphic or trimorphic, — gastrozooids, dactylozooids, and 

 sometimes medusoid reproductive buds. 



The type of Stylaster gemmascens came from the Indian Ocean The North 

 Atlantic forms identified with the type were dredged by the "Lightning" and 

 the "Porcupine " (530 fathoms). Another record is given by Sars (Fork. Selskabs 

 Christiania, 1872, p. 115), from a great depth in the Foldenfjord, Norway. 



[Issued separately, 18th February 1910.) 



