PART 5 



A MONOGRAPH OF THE EXISTING CRINOIDS 



633 



Diagnostic features. The cirri are XXV-LX, 26-43, the longest segments hardly, 

 if at all, longer than wide; PI is up to 10.5 mm. long with 10 to 17 segments while P^ is 

 usually 1.5 to 2.0 mm. shorter with 9 to 14 segments and not more than 8.5 mm. long; 

 P 5 is usually the first genital pinnule. 



Description [modified by A.M.C.]. The centrodorsal is rounded conical, broader 

 than high, with a smooth rounded dorsal pole and with the rim extended into prominent 

 interradial angles. The cirrus sockets are rather closely set, irregularly arranged or 

 partly forming irregular rows. 



The cirri are up to LX, 26-43. The component segments are short, the fourth 

 to tenth being somewhat longer than the remainder, about as long as broad, sometimes 

 a little longer. Beyond the middle of the cirri the segments develop a dorsal carination, 

 the profile of which is more or less sharply convex; this almost disappears on the outer- 

 most segments. The opposing spine is erect. The terminal claw is short and strongly 

 curved. The peripheral cirri are turned upward, protruding between the arms over 

 the disk. 



The radials form a narrow band beyond the run of the centrodorsal. The IBri 

 are more than twice as broad as long with the lateral borders parallel and the distal 

 edge incised. The IBr 2 (axillaries) are a third again as broad as long, rhombic, with 

 the lateral angles truncated and all the sides concave. The IBr series and lower 

 brachials are in lateral contact and have sharply flattened sides. 



The distal edges of the outer brachials are smooth but somewhat thickened, 

 giving the arms a serrate profile. 



Syzygies normally occur between brachials 3+4 and 9 + 10 and distally at intervals 

 of from 2 to 6 (usually 3 or 4) muscular articulations, but there are often irregularities, 

 even in the positions of the first two syzygies. 



FIGURE 36. Isomctra ?vivipara Mortensen, B.M., 1938.12.7.121, Discovery Investiga- 

 tions station 175, Bransfield Strait: a, Cirrus; b, P,; c, Pj. /. vivipara, B.M., 

 1938.12.7.124-5 and 126-133, stations W.S. 81 and W.S. 83: d, PS of 9; t and/, 

 lateral and distal views of PS of cf . 



