PART 5 A MONOGRAPH OF THE EXISTING CRINOIDS 833 



SIBOGACRINUS ANOMALUS (A. H. Clark) 



[See vol. 1, pt. 1, fig. 414, p. 319] 



Atelecrinus anomalus A. H. CLARK, Notes Leyden Mus., vol. 34, 1912, p. 153 (description; Siboga 

 sta. 177); Unstalked crinoids of the Siboga-E\ped., 1918, p. viii (a new type [of Atelecrinus] 

 discovered by the Siboga), p. 266 (in key; range), p. 269 (detailed description; sta. 177); fig. 17, 

 p. 269, p. 274 (listed), pi. 26, fig. 96. GiSLfitf, Zool. Bidrag. Uppsala, vol. 9, 1924, p. 166 (com- 

 parison with fossil Jaekelometra); Ark. Zool., vol. 19, No. 32, 1928, p. 11. 



Description. The centrodorsal is very long, about 1.8 mm. broad at the base 

 and 3 mm. long in the radial line, cylindrical in the proximal third, from that point 

 onward conical, the transition from the cylindrical to the conical portion being rather 

 abrupt. The cirrus sockets are moderate in size, and are arranged in 10 evenly spaced 

 columns, 3, rarely 2 or 4, to a column. Each socket is separated from its neighbors 

 in the same column by about the same distance that the columns are apart. The 

 surface of the centrodorsal is smooth and undifferentiated. The cirrus sockets are 

 simple excavations, without the raised borders found in the species of Atelecrinus. 

 In each cirrus socket, just proximal to the middle, a half conical ridge extends in from 

 either side; these ridges are triangular in outline, the apex of the triangle abutting on the 

 central canal. The ligament areas ventral to these ridges are rounded proximally, 

 the sides converging in a sharp angle at the central canal; their area is approx- 

 imately equal to that of the ridges. The ligament areas lying dorsally from (or 

 distal to) these ridges are shallower, occupying about half the area of the cirrus socket 

 or rather more. The interradial portions of the centrodorsal just below the basals are 

 slightly raised above the general surface so that a section of the centrodorsal through 

 the base (or proximal third) is rounded pentagonal. The cirrus sockets occur almost 

 exclusively on the conical distal two-thirds of the centrodorsal. 



The cirri are XXX, 19-20, about 13 mm. long. The first two segments are about 

 twice as broad as long, the third is slightly broader than long, or about as long as broad, 

 the fourth is twice as long as the diameter of its ends, the fifth is nearly or quite 3 times 

 as long as the width of its ends, and the sixth is slightly shorter. The following seg- 

 ments decrease gradually in length so that the fourteenth and following are only slight- 

 ly longer than broad; the fourth to seventh are moderately constricted centrally, but 

 this central constriction disappears in the next 2 or 3 succeeding, when the cirri become 

 strongly compressed laterally. The opposing spine is terminal, small, and blunt. 

 The terminal claw is slightly longer than the penultimate segment, rather stout, evenly 

 tapering, and rather strongly curved. 



The basals are very large and conspicuous, broadly 7-sided, in contact laterally 

 for a distance equal to about half the greatest (median) length. The proximal edge 

 of the basals bordering the swollen interradial portion of the centrodorsal is slightly 

 concave. The two adjacent proximal edges are of about the same length, but straight. 

 The anterior angle of the basals is broadly obtuse and is of the same degree as the mid- 

 radial angle made by the proximal edges of adjacent basals over the proximal edge of 

 the centrodorsal. The sides of the basal ring are parallel, with the result that the 

 basal ring continues the column made by the columnar basal third of the centrodorsal. 



The radials are slightly broader than long, in lateral contact throughout their en- 

 tire length; their interradial angles are somewhat produced. Proximally the radials 

 are slightly rounded dorsally, becoming more strongly rounded distally, especially 



