PART 5 A MONOGRAPH OF THE EXISTING CRIXOIDS 509 



Diagnostic features. — The centrodorsal, which is wholly without spiues, is more 

 than half again as high as broad at the base, and its radial areas carry from three to four 

 columns of cirrus sockets of which about a hundred probably bore cirri ; the spines on the 

 ossicles of the division series and on the brachials are short, and are confined to the 

 borders of the ossicles, and there is a slight development of synarthrial and articular 

 tubercles; Pj and P2 have 17 and 15 segments respectively. 



Description.- — The centrodorsal is elongate-conical, 8.5 mm. high by 5 mm. broad 

 at the base. For about the basal third the sides converge somewhat less rapidly than 

 from that point onwards. The tip is unusually acute. The sides are divided into five 

 radial areas bj' bare interradial lines, which are not raised above the general surface 

 of the centrodorsal except in one interradial area, where the median portion of the 

 bare lines forms a low and inconspicuous ridge. These lines are at first from half 

 as wide to quite as wide as the adjacent cirrus sockets, but gradually narrow and 

 become obsolete somewhere between the basal half and terminal third of the centro- 

 dorsal. Two of the radial areas contain basally four closely crowded columns of cirrus 

 sockets, the sockets of one column alternating with those of the columns on either 

 side; between a third and half way up the centrodorsal these four columns of cirrus 

 sockets pass over into three. Another radial area has basally three columns, and along 

 the right hand border a few sockets representing a fourth column, and the two re- 

 maining radial areas have three columns of sockets from base to apex. There are 1.3 

 or 14 sockets to a column, of which the basal four to sLx, which are larger and better 

 formed than the others, alone appear to have been functional. But well-formed 

 sockets continue to the apex of the centrodorsal, which is wholly without spines. 

 [Note by A.M.C] The total number of cirri is estimated at about XCV, though the 

 sockets altogether number about 140. 



None of the cirri are preserved beyond the base. The longest slump remaining 

 consists of three segments which do not differ from those in the largest cirri of S. 

 triserialis. 



The ends of the basal rays are prominent as tubercles in the interradial angles 

 bridging over the regions between the shallow subradial clefts. 



The surface and distal border of the radials are wholly without spines. The 

 division series and arms resemble those of S. triserialis, but are proportionally stouter 

 and also more rugged, with a slight development of synarthrial and articular tubercles. 

 The general effect is much less spinous than S. triserialis. The spines on the proximal 

 and distal edges of the elements of the division series and lower oblong brachials, 

 though very numerous, are short. The lateral borders of the axillaries aiul of the 

 second brachials bear a series of webbed spines which are much longer than those 

 found elsewhere. The outer brachials have a group of long and verj' closely set spines 

 in the middle of the distal border, but much shorter spines elsewhere. 



The width at the first syzygy is 1.8 mm. and the length to the second syzygy 14 

 mm. 



P, is 8 mm. long with about 17 segments, of which the first is nearly half again 

 as long as the maximum width, the second and third are somewhat shorter, the fourth 

 is half again as long as broad and the remainder are greatly elongated. The pinnule 

 is slender, gradually tapering, and becomes filiform in the outer portion. The first 

 three segments have the corners on the side turned toward the arm very broadly 

 rounded. 



