66 BULLETIN 82, UNITED STATES NATIONAL MUSEUM 



The IBr 2 (axillaries) are pentagonal with the lateral border shorter than that of the 

 IBrj. A strong synarthrial tubercle is present on the IBr series. 



The 10 arms are about 50 mm. long in one specimen. The first brachials are in 

 contact. There is a strong synarthrial tubercle on the articulation between the first 

 two brachials. From the third to about the seventh each brachial forms with the 

 following on the distal edge dorsally a laterally placed tubercle, succeeding tubercles 

 alternating in position. The following brachials are wedge-shaped, and are provided 

 dorsally on the distal border with alternating sharp angles which on the outer brachials 

 gradually become blunt. 



The first syzygy is between brachials 3+4, the second is between about brachials 

 9 + 10, and the distal intersyzygial interval is, with fair regularity, 4 or 5 muscular 

 articulations. 



The three lowest pinnules on either side of the arm are very similar. They are 

 slender, stiff, and sharp with elongated segments, but with a very broad base and 

 prominent angles on the dorsal side. P! is about 8 mm. long and is composed of about 

 12 segments. P 2 is slightly shorter and is composed of about 10 segments. P 3 is 

 markedly shorter and, like P 2 , is composed of about 10 segments. The first segment 

 of all these pinnules is very broad, markedly broader than long. The second segment 

 is not broader than long. The third is somewhat longer, but somewhat more slender 

 than those preceding. The succeeding segments progressively decrease in width, 

 though only slowly in length. P 4 is only 4 mm. long; its component segments are all 

 broader than long. From P 8 onward the pinnules acquire a markedly larger number of 

 segments, about 18, and those in the proximal half are about equally broad. In these, 

 the genital pinnules, which are markedly thicker than the first pinnules and longer 

 than P 4 , the ambulacral surface shows a very prominent calcareous plating that 

 consists of plates arranged more or less regularly in rows on both sides of the ambulacral 

 groove. 



The disk is naked and is about 7 mm. in diameter. 



The color is whitish. From the IBr, there run outward on the dorsal side of the 

 axillaries and for some distance along the arms two pale purple stripes; a bowed line 

 of similar color occurs in the interambulacral areas of the disk, while the anal tube is 

 marked with a number of almost black narrow longitudinal stripes. 



Doderlein said it cannot be doubted that these specimens represent Antedon 

 bidens, which was described by Bell from Torres Strait. He remarked that the single 

 essential difference between his specimens and Bell's description and figure is that Bell 

 gives all the segments of the lowest pinnules as much longer than broad, while in his 

 specimens this is the case only from the third segment onward. Doderlein remarked 

 that Carpenter did not place Antedon bidens in any of this specific groups, but listed 

 it among the unclassified species at the end of his discussion of the several groups 

 including 10-armed species of Antedon. He said that this species might well be 

 looked for under the Tenella group, perhaps also under the Acoela group because of 

 the plating of the pinnules. He remarked that he had been unable to discover any 

 close relationship between Antedon bidens and any of the species in these groups. 



A very small specimen with arms only 8 mm. long collected by the Gazelle in 

 northwestern Australia appears to belong to this species as redescribed by Professor 

 Doderlein, but it is so small that positive identification is not possible. 



