36 THE VOYAGE OF H.M.S. CHALLENGER. 



Sporadopora, Moseley. 



This genus, hitherto unknown, I described in the Proceedings of the Royal 

 Society, vol. clxxiii., 1876, p. 94, under the name Polypora, but as I was informed by 

 Mr Etheridge, junior, that the name Polypora was already in use, I changed it to 

 Sporadopora, which refers to the irregular scattering of the pores over the surface of the 

 ccenosteum. The genus is founded on a single species, Sporadopora dichotoma, obtained 

 on one occasion only by the Challenger off the mouth of the Eio de la Plata in 600 

 fathoms. 



Ccenosteum of Sporadopora dichotoma. 



The ccenosteum or hard calcareous structure in this Hydroid occurs in the form of stout 

 upright stems, which branch with tolerable regularity dichotomously to form a flabelli- 

 form expanse. The stem is usually nearly circular in section towards its base, but 

 becomes compressed above in the plane of the fan, whilst the branches and branchlets 

 forming the fan itself are very much flattened, so as to be more oblong than oval in 

 transverse section. The number of branchings is few, only four or five at most. The 

 flattened branches and branchlets coalesce at their adjacent margins. A figure of a 

 well-grown but partly broken example is given on Plate I. fig. 2, reduced to half the 

 natural dimensions. 



Sometimes the stems are somewhat bent and irregular, as are also the flabellate 

 expanses which they support. The height of the largest specimen obtained is about 5| 

 inches, and the breadth of the fan about 5 inches. The diameter of the stem at its base 

 is about 1 inch ; in more slender specimens h inch to f inch. In one large broken and dead 

 specimen the stem is 2 inches in diameter. The ccenosteum is dense and heavy, and when 

 macerated out from a living specimen is of a pearly white, and smooth and glistening 

 in appearance (PL I. fig. 1). The surface is pierced by deep pores, which are simply 

 circular in outline and of two kinds, large and small, and are scattered irregularly over 

 it. The larger pores or gastropores are less numerous than the smaller. They are deep, 

 reaching nearly to the central axis of the branch or stem on which they are situated, and 

 contain a deep-seated, long, and slender style. The smaller more numerous pores, the 

 dactylopores, are thickly dispersed between the larger ones. They have no style. The 

 pores are usually more abundant on one face of the coral flabellum than on the other ; 

 indeed, large areas of what may be called the back of the stem are often devoid of pores 

 altogether. 



The appearance of the surface of the ccenosteum as seen by reflected light under a low 

 magnifying power is shown in Plate II. fig. 2. The surface presents slight irregular 

 undulations. Its texture is somewhat like that of loaf sugar, being composed of closely 



