A MONOGRAPH OF THE EXISTING CRINOIDS 201 



Decametra studeri A. H. CLARK, Die Fauna Sudwest-Australiens, vol. 3, Lief. 13, 1911, p. 441 (Aus- 

 tralian tropical species occurring south to Dirk Hartog Island), p. 446 (summary of west 

 Australian records); Mem. Australian Mus., vol. 4, 1911, p. 724 (locality), p. 774 (synonymy; 

 characters; Australian record); Proc. U. S. Nat. Mus., vol. 43, 1912, p. 384 ( = 0iigometra 

 studeri), p. 400 (locality); Crinoids of the Indian Ocean, 1912, p. 164 (synonymy; locality); 

 Internat. Rev. gesamt, Hydrobiol. und Hydrogr., 1915, p. 225 and following (detailed account 

 of the distribution in Australia). HAKTMBYER, Mitt. Zool. Mus. Berlin, vol. 8, No. 2, 1916, 

 p. 235 (Dirk Hartog Island; No. 6381). A. H. CLARK, Unstalked crinoids of the Siboga- 

 Exped., 1918, p. 118 (in key; range). H. L. CLARK, Echinoderm fauna of Australia, 1946, p. 52. 



Diagnostic features. The cirri are 7 mm. long with 16-18 segments of which the 

 sixth or seventh and following are about as long as broad and the four to six preceding 

 the penultimate bear a single median dorsal tubercle or short spine; P 2 is the largest 

 and longest pinnule, 6 mm. long with 18 segments; P 3 is shorter than PI; the arms are 

 45 mm. long. 



. Description. The centrodorsal is thin discoidal, with the bare polar area flat. 

 The cirrus sockets are arranged in a single marginal row. 



The cirri are XII, 16-18, 7 mm. long. The first segment is very short, the fol- 

 lowing gradually increasing in length to the sixth or seventh which, with the remainder, 

 is about as long as broad. On the third or fourth segment a low transverse ridge is 

 developed on the dorsal side which is subterminal in position. This gradually moves 

 proximally, after 3 or 4 segments attaining a median position, at the same time 

 gaining slightly in height; distally it progressively decreases in width, and on the last 

 4 segments before the penultimate becomes a low median spine. The ridges on ah 1 

 the segments appear as low spines in lateral view. The opposing spine is median 

 in position, and arises from the entire dorsal surface of the penultimate segment. It 

 is much higher than the short processes on the preceding segments, its height equaling 

 about half the width of the penultimate segment. The terminal claw is rather 

 stout, slightly longer than the penultimate segment, abruptly curved basally but 

 becoming more nearly straight distally. 



The radials are concealed by the centrodorsal in the midradial line, but are visible 

 in the interradial angles of the calyx. The IBrj are oblong, short, about 4 times as 

 broad as long, with the lateral edges straight. The IBr 2 (axillaries) are broadly pen- 

 tagonal, almost triangular, about twice as broad as long. Synarthrial tubercles are 

 slightly developed. 



The 10 arms are 45 mm. long. The first 2 brachials are wedge-shaped, slightly 

 over twice as broad as the exterior length, the first ulteriorly united for rather more 

 than the proximal half, diverging at an obtuse angle distally. The first syzygial 

 pair (composed of brachials 3 + 4) is slightly longer interiorly than exteriorly, about 

 twice as broad as the median length. The following 3 brachials are oblong, about 

 3 times as broad as long, the brachials then becoming obliquely wedge-shaped and 

 after the twelfth triangular, slightly broader than long, and hi the terminal portion 

 of the arm wedge-shaped and about as long as broad. After the tenth the brachials 

 develop rather prominent and slightly overlapping distal ends, but this feature grad- 

 ually dies away after about the middle of the arms. 



Syzygies occur between brachials 3 + 4, 9+10 (sometimes omitted), and 14 + 15, 

 and distally at intervals of 4-7 (usually 5) muscular articulations. 



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