216 BULLETIN 82, UNITED STATES NATIONAL MUSEUM 



MONACHOMETRA ROBUSTA (P. H. Carpenter) 



Antedon robusta P. H. CARPENTER, Challenger Reports, Zoology, vol. 26, pt. 60, 1888, p. 220 (descrip- 

 tion; Challenger station 192), pi. 44, fig. 1. HARTLAUB, Bull. Mus. Comp. Zool., vol. 27, No. 4, 

 1895, p. 134 (size compared with that of A. [Thalassometra] agassizii). HAMANN, Bronn's 

 Klassen und Ordnungen des Tier-Reichs, vol. 2, Abt. 3, 1907, p. 1581 (listed). A. H. CLARK, 

 Crinoids of the Indian Ocean, 1912, p. 34 (identity). HARTLAUB, Mem. Mus. Comp. Zool., 

 vol. 27, No. 4, 1912, p. 309 (in Spinifera group; history). 



Antedon patula (part) BELL, Journ. Linn. Soc. (Zool.), vol. 24, 1893, p. 341 (Sahul Bank). 



Charitometra robusta A. H. CLARK, Smithsonian Misc. Coll., vol. 50, pt. 3, 1907, p. 361 (listed). 



Pachylometra robusta A. H. CLARK, Proc. Biol. Soc. Washington, vol. 22, 1909, p. 21 (listed), p. 82 

 (compared with P. macilenta); Crinoids of the Indian Ocean, 1912, p. 34 (identity), p. 220 

 (synonymy; locality); Smithsonian Misc. Coll., vol. 61, No. 15, 1913, p. 49 (published references 

 to specimens in the B. M.; Challenger station 192; Sahul Bank). 



Perissometra robusta A. H. CLARK, Journ. Washington Acad. Sci., vol. 6, No. 17, 1916, p. 607 (listed); 

 Unstalked crinoids of the Siboga-Exped., 1918, pp. 176, 178 (in key; range), p. 179 (references; 

 notes; Sta. 297), p. 275 (listed). GISLEN, Kungl. Fysiogr. Sallsk. Handl., new ser., vol. 45, 

 No. 11, 1934, p. 20. 



Monachometra robusta GISLEN, Ark. Zool., vol. 19, No. 32, Feb. 20, 1928, p. 9, No. 40 (notes). 



Diagnostic features. The brachials in the proximal portion of the arms are smooth, 

 without raised distal edges; the centrodorsal is low, thick discoidal, with the cirrus 

 sockets arranged in 15 closely crowded and somewhat irregular columns; the 12-17 

 arms are up to 250 mm. long; and the cirri are up to 70 mm. in length with up to 31 

 segments. 



Description. The centrodorsal is a thick disk 9 mm. in diameter. 



The cirri are about L, 18-31 ; according to the figure all the segments except the basal 

 are usually longer than broad, although the last two or three may be about as long as 

 broad. The longest segments are from one-third to one-half again as long as broad. 

 There is a small opposing spine. 



The radials are just visible in the angles of the calyx above the low interradial 

 processes of the centrodorsal, which partly conceals the short IB^ in their median line. 

 The IBr 2 (axillaries) are widely rhombic with an obtuse distal and sharper proximal 

 angle, the latter rising to form a prominence (synarthrial tubercle) with the IBri. The 

 IIBr series are 2, one being present on each of two postradial series. 



The 12 arms are nearly 250 mm. long and are composed of more than 200 brachials. 

 The first two brachials (or the elements of the IIBr series) bear a synarthrial tubercle 

 like the IBr series, and have much flattened outer sides like the axillaries. The inner 

 sides of the second brachials and both sides of the first syzygial pair are also sometimes 

 flattened. The first few brachials are oblong, the later more triangular. 



Syzygies occur between brachials 3+4, the next not until after brachials 21 + 22; 

 the distal intersyzygial interval is long, from 10 to 26 oblique muscular articulations. 



PI is about 12 mm. long and is composed of about 20 segments the lowest of 

 which are much larger than their successors and are of almost prismatic shape, being 

 flattened against the corresponding segments of the adjacent pinnules. P a is similar 

 but slightly smaller, and its successors are of about the same length, but have broader 

 and flatter segments. The later pinnules gradually become elongated, but none of 

 the segments are conspicuously wider than the rest. In the styliform middle and 

 outer pinnules the first segment is flattened and expanded, with a curved distal edge. 



The disk is 12 mm. in diameter, thickly plated, as are also the arms, both along 



