214 BULLETIN 82, UNITED STATES NATIONAL MUSEUM 



vol. 1, Heft 1, pp. 211 and following (syzygies; regeneration).— Hamann, Bronn'a Klassen und 

 Ordnungen des Tier-Reichs, vol. 2, Abt. 3, 1907. pp. 1578, 1581 (listed).— Hartlaub, Mem. 

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



Charitometra flexilis A. II. Clark, Smithsonian Misc. Coll., vol. 50, pt. 3, 1907, p. 361 (listed). 



Pachylometra flexilis A. H. Clark, Proc. Biol. Soc. Washington, vol. 22, 1909, p. 21 (listed); Proe. 

 U. S. Nat. Mus., vol. 39, 1911, p. 557 (correction of the dimensions given in the original descrip- 

 tion); Proc. Biol. Soc. Washington, vol. 25, 1912, p. 77 (comparison with P. crassa); Crinoids of 

 the Indian Ocean, 1912, pp. 33, 34 (identity), p. 219 (synonymy; locality); Smithsonian Misc. 

 Coll., vol. 61, No. 15, 1913, p. 48 (published references to specimens in the B. M.; Challenger 

 station 192). 



Perissometra flexilis A. II. Clark, Journ. Washington Acad. Sci., vol. 6, No. 17, 1916, p. 607 (listed); 

 Unstalked crinoids of the Siboga-Exped., 1918, p. 178 (in key; range), p. 182 (references).— 

 GiSLfiN, Ark. Zool., vol. 19, No. 32, Feb. 20, 1928, p. 8, No. 37 (notes). 



Diagnostic features. — The brachials in the proximal portion of the arms have 

 unmodified distal edges; the centrodorsal is columnar or slightly tapering with the 

 cirrus sockets arranged in 10 columns, the midradial lines bare; the 10-13 arms are 

 about 175 mm. long; and the cirri have 20-25 segments. 



Description. — The centrodorsal is columnar or slightly tapering with its ventral 

 angles produced into marked interradial processes. The cirrus sockets are arranged in 

 10 columns which are in contact interradially but separated by a bare line in the 

 midradial areas; there are 3 or 4 sockets in a column. 



The cirri are XXX-XXXV, 20-25. The segments are stout but very smooth, 

 and are mostly longer than broad. In the figure they are shown as from very slightly 

 longer than broad to about one-third again as long as broad. The penultimate segment 

 bears an opposing spine. 



The radials are visible. The radials and the elements of the IBr series, especially 

 the IBr 2 (axillaries) are rather sharply convex with a more or less distinct median 

 tubercle. The IBr axillaries are subhexagonal and considerably broader than the 

 IBr t which arc short and bandlike and in close contact laterally. The IIBr series, 

 when present, are 2. 



The 10-13 arms are about 175 mm. long. The first two brachials or the elements 

 of the IIBr series, when these are present, have median tubercles like those of the IBr 

 series, but less prominent. The IBr axillary and the two following segments have 

 sharp straight edges and flat wall-like sides. The inner side of the second and third 

 (hypozygal) brachials are also flattened, especially when the IIBr series are absent. The 

 lower brachials are somewhat discoidal with very rounded surfaces, and those following 

 are shortly triangular with the edges slightly raised, but they gradually become more 

 smooth and relatively longer. 



Syzygies occur between brachials 3 + 4, again from between brachials 13 + 14 to 

 between brachials 19+20, and distally at intervals of from 4 to 8, usually 5 or 6, oblique 

 muscular articulations. 



The first piimules reach 13 mm. in length and are composed of about 40 short and 

 broad segments; their outer sides are somewhat llattcned against one another below. 

 The pinnules following arc of about the same length, but the segments gradually 

 increase in size and diminish in number, still remaining much broader than long. In 

 the pinnules of the sixteenth and of several of the following brachials the fourth and 

 fifth segments are considerably broader than their fellows, but in the later pinnules 

 the segments are longer than broad. 



