260 BULLETIN 82, UNITED STATES NATIONAL MUSEUM 



Perissometra macilenta A. H. 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. 181 (references); Journ. 

 Linn. Soc. (Zool.), vol 36, No. 249, April 1919, p. 658 (south of Timorlaut; 500 fathoms; notes). 

 GiSLfiN, Kungl. Fysiogr. Sallsk. Handl., new ser., vol. 45, No. 11, 1934, p. 21. 



Perissometra occidentalis A. H. CLARK, Journ. Linn. Soc. (Zool.), vol. 36, No. 249, April 1929, p. 635 

 (listed), p. 655 (120 miles off the Seychelles, 800 fathoms; description), pi. 42, fig. 10; John Murray 

 Exped., 1933-34, Sci. Reports, vol. 4, No. 4, Crinoids, 1936, 1937, p. 101 (range), p. 104. 



Diagnostic features. The cirri, which are arranged in 10 columns on the centro- 

 dorsal, are comparatively long and slender, 25-36 mm. long with 15-22 segments of 

 which the earlier are from one-third to half again as long as broad and the distal are 

 about as long as broad; the 13-15 arms are 130-170 mm. long; and the division series 

 and arm bases are rugose. 



Description. The centrodorsal is a short truncated cone, moderately large, with 

 the bare dorsal pole flat and 2.5 mm. in diameter. The cirrus sockets are arranged in 

 10 columns of usually 2 each, the columns of adjacent radial areas closely crowded 

 and more or less alternating, the two within the same radial area usually slightly sepa- 

 rated proximally. 



The cirri are XX, 21-22, from 25 to 36 mm. long, comparatively long and slender. 

 The first segment is very short, the second is twice as broad as long, the third is about 

 as long as broad, and those following gradually increase in length to the sixth, which 

 is about half again as long as broad. The next two or three segments are similar, those 

 succeeding very gradually decreasing in length so that the segments in the terminal 

 third of the cirri are about as long as broad. The antepenultimate segment is about 

 one-third again as long as broad, and the penultimate segment is half again as long as 

 broad and is slightly less in diameter than the preceding. The opposing spine is very 

 small, though prominent; it is terminally situated and directed obliquely forward. 

 The terminal claw is about as long as the penultimate segment, stout basally but 

 becoming slender in the distal half, and is moderately curved. The cirri are moderately 

 compressed in the distal two-thirds. The two or three segments before the penultimate 

 have slight traces of terminal dorsal tubercles. 



The ends of the basal rays are visible as large rhombic tubercles in the angles of 

 the calyx. 



The radials are visible only as a large transversely oval tubercle between the 

 centrodorsal and the IBr^ The IB^ are very short, arcuate, in close lateral apposition; 

 the proximal border is more or less crenulate and bears a large and rather high trans- 

 versely oval median tubercle. The IBr 2 (axillaries) are rhombic, very short, somewhat 

 over twice as broad as long, with the edges somewhat cremdate and somewhat pro- 

 duced, the proximal imbricating more or less over the distal border of the IBrij the 

 lateral edges are very short and are in close apposition. The IBr 2 bear a rather high 

 rounded median tubercle. The IIBr series are 4(3+4), in close lateral apposition and 

 sharply flattened, with the lateral borders somewhat everted. The distal edge of the 

 IIBr, is somewhat everted, and the proximal edge of the IIBr 2 is more strongly everted, 

 the latter imbricating over the former except in the median line. The IIBr, usually 

 bears a strong rounded median tubercle like that on the two components of the IBr 

 series. 



The 13 arms in the type specimen are 170 mm. long, elongated and comparatively 

 slender, resembling those of Pachylometra robusta. 



