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



2. Antedon disciformis, n. sp. (PI. IV. figs. 2, a-d ; PI. XXXIX. fig. 4). 

 Specific formula — A.2.-J-. 



Centro-dorsal a thick pentagonal disk, with an irregular row of marginal cirri and the 

 dorsal surface free. Fifteen to twenty cirri, of twenty-five to thirty joints, several of 

 which are longer than wide. The fourth or fifth joints project beyond their successors 

 on the dorsal side, and the following joints gradually develop a sharp forward-projecting 

 spine at their distal edge ; as the joints shorten this comes to be placed further and 

 further back, and is both shorter and more upright. 



First radials mostly concealed ; the second oblong and quite free laterally ; axillaries 

 pentagonal, nearly twice their length. The rays are well separated and may divide 

 twice. Two distiehals ; the first nearly oblong, and the axillary without a syzygy. 

 Fifteen to twenty arms of about one hundred and twenty smooth and rounded joints, 

 the first few discoidal and their successors triangular, about as wide as long, gradually 

 becoming more quadrate. A syzygy in the third, and then between the eighth and 

 fourteenth brachials ; others at intervals of one to five joints. 



The second brachial has a slender pinnule of about eighteen short joints, but little 

 longer than wide ; but the third brachial has no pinnule. The next pair are rather 

 stouter and much longer than the first pinnule, reaching 12 mm., and consisting of 

 twenty elongated joints, the apposed edges of which are somewhat produced towards the 

 ventral side. The pinnule on the sixth brachial may be of nearly equal size or distinctly 

 smaller, and its successors diminish in length to about the tenth brachial, and then 

 increase, becoming exceedingly slender in the outer parts of the arms. 



Disk naked and rather incised, with a few sacculi, which are very abundant along 

 the ambulacra of the arms and pinnules. 



Colour in spirit, — the skeleton almost white, and the perisome grey or brownish. 



Disk about 8 mm.; spread probably 15 cm. 



Locality. — Zebu Reefs. Six specimens and one fragment. 



Remarks. — This species, while resembling Antedon manca in its spiny cirri and in 

 the absence of a pinnule on the third brachial, differs from it altogether in having no 

 palmar axillary, and in the constant presence of a pinnule on the second brachial. The 

 rays being quite free laterally, it stands rather near to Antedon marginata (PI. XL.), 

 resembling it also in the elongated joints, in the great size of the pinnules on the fourth 

 and fifth brachials, and in the absence of a palmar axillary, though sharply distinguished 

 from it by the want of a pinnule on the third brachial and by the very spiny cirri. 



The extreme flatness of the centro-dorsal and the limitation of the cirri to its margin, 

 so as to leave the dorsal surface free (PI. IV. fig. 2a), recall the characters of Actinometra ; 



