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



joints, all of which are perfectly smooth and without any traces of the dorsal projections 

 which are so characteristic of the joints composing the shorter and more centrally placed 

 cirri. These long marginal cirri eventually develop spines and only differ in their 

 greater length from those nearer the dorsal pole. 



Besides seven individuals of Antedon australis found at Station 150, which are all 

 pretty equally developed, the Challenger also obtained a mutilated calyx of much smaller 

 size, from which all the arms had broken away at the syzygies in the third brachials 

 (PL XXVI. fig. 5). It may belong to this same species, but if so, it is remarkable in not 

 showing more of the first radials externally than is visible in the more mature forms, 

 though, on the other hand, the axillaries are relatively longer, as would be expected in a 

 young individual. This may also account for the elongated shape of the joints of the 

 first pinnule, and for the present it will be safest to regard this form, which has 

 numerous cirrus-sockets on the centro-dorsal, though barely 4 mm. in diameter, as a 

 young example of Antedon australis. 



4. Antedon rhomboidea, n. sp. (PL XII. figs. 1,2; PL XXIV. figs. 1-3). 



Specific formula — A.y. 



Description of an Individual. — Centro-dorsal hemispherical, with a flattened and 

 cirrus-free dorsal pole. Sixty or more cirri, of thirty to thirty-five segments, several of 

 which are longer than wide. The middle joints project beyond their successors on the 

 dorsal side, and in the shorter terminal joints this projection develops into a blunt keel. 



First radials just visible ; the second trapezoidal, rather deeply incised and rising 

 to tubercles at the junction with the rhombic axillaries, which are much wider than the 

 distal ends of the second radials. 



Ten arms, with a median knob at the junction of the first two brachials, and others 

 alternating on the outer and inner sides till the ninth or tenth joint. The first brachials 

 deeply incised, with very short inner ends, which barely meet above the sharp angles of 

 the axillaries. Middle arm -joints triangular, soon becoming quadrate, as long as or 

 longer than wide, and slightly overlapping. Syzygies in the third, eighth, and fourteenth 

 brachials, and then at intervals of two or three joints. The first two pinnules on each 

 side about 22 mm. long, flagellate, and composed of fifty short joints, the basal 

 ones broad, flattened, and slightly carinate, and the later joints serrate. The third 

 pinnule of about the same size but with fewer joints, the basal ones being stouter and 

 the middle ones longer than wide. The following pinnules are more massive, with square 

 joints, which soon become elongated, the two basal ones but little modified. 



Mouth subcentral, with rather large calcareous concretions round the peristome and 

 at the sides of the ambulacra on the disk and arm-bases. Sacculi very abundant. 



Colour in spirit, — light brownish-white. 



