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



ribs, but with large rough warts. Only those of the last row higher than broad, 

 with longer teeth on the end. Height to breadth in mm. — 0-42-0-33 ; 0-26-0-4 ; 

 0-24-0-46. 



Opercular spicules. — Pointed, triangular, hollowed out in the middle line, with two 

 lateral combs, 0-4-0-15 ; 0-33-015 mm. 



Coenenchyma — spicules of the upper layer, irregularly three- or four-sided. Height 

 to breadth, 0-25-0-3 ; 0-3-0-47 ; 0-19-0-33 ; 0-3-0-22 ; 0-29-0-25 mm. Lower layer, 

 irregularly polygonal with a few small warts, 0-28-0-28 ; 0-23-0-2 ; 0-3-0-16 mm. 



Habitat. — Station 135d, off Inaccessible Island, Tristan da Cunha ; depth, 55 to 70 

 fathoms. 



6. Thouarella variabilis, n. sp. 



Main stem simple. The twigs come off from the main stem in three directions at 

 nearly right angles, they are thin, flexible and frequently branched. The polyps are 

 placed on the twigs in short spirals of always three in number. The calycles are cup- 

 shaped, 2 to 2-5 mm. long, covered with only a few scales, forming four to five transverse 

 rows. The scales of the uppermost row are produced into longer or shorter spines, which 

 surround the calyx wall. The operculum consists of eight concave scales, which vary 

 slightly according to position. When closed it forms a low, slightly projecting cone, 

 the apex of which is generally overtopped by the spicular spines. The scales of the 

 coenenchyma are large, and their edges overlap laterally. 



This species, of which there are numerous examples, varies to an extraordinary 

 degree in the size of the calyces, the development of the spines, and the development of 

 the colony, without it being possible thereby to sharply separate the individual forms 

 specifically. Nevertheless one can generally distinguish the following three varieties 

 from each another. 



Var. a. The type (PI. XIV. figs. 1,2; PI. XXI. fig. 1). 



The commonest form. The main stem is simple and in the largest specimen 

 attains a height of 300 mm. The base is wanting in all the specimens. The axis 

 is in its lower part firm and brittle, and consists of a layer of concentrically placed, 

 calcified horny lameUse and a softer medulla ; above, the axis becomes softer and 

 flexible. 



Its transverse section is elongatedly oval, the largest transverse diameter, in the 

 largest piece, reaches near the base 3 mm., in the upper third 1'5 mm. The entire 

 stem exhibits a spiral twisting round its axis. The colour is brownish-yellow. The 

 coenenchyma is very thin and contains only a single layer of thin calcareous scales. 

 The thin twigs, all the way up, come off from the stem mainly in three directions, at 



