MONOGRAPH OF THE EXISTING CRINOIDS. 175 



Only in the species of Zygometridse do we find the proximal syzygies con- 

 fined to the articulation between the elements of the IBr series, and only in this 

 group do they occur in all the species. 



In the species of the genus Comaster, excepting in C. multifida and 0. taviana 

 in which they are entirely ab- 

 sent, the proximal syzygies reach 

 a high state of perfection. In 

 the articulation between the os- 

 sicles of the IBr series in '. fru- 

 ticosus (part 1, fig. 37, p. 75) 

 the central canal is surrounded 

 by a well-marked rim from which 

 a more or less broad median ridge 

 runs to the middle of the ventral 

 border of the joint face; be- 

 tween the rim and the middle of 

 the dorsal border of the joint 

 face are half a dozen or so small 

 narrow ridges of which the two 

 central usually cover the entire 

 distance while the others radiate, 

 more or less irregularly, outward 

 from the central canal toward the 

 dorsal and dorsolateral border. 

 The outer half of the joint face 

 is marked by usually three or 

 four more or less prominent con- 

 centric ridges ; between this outer 

 concentrically arranged portion 

 and the rim about the central 

 canal the surf ace of the joint face 

 is thickly set with blunt points 

 and tubercles and short or inter- 

 rupted ridges of various lengths 

 which may be arranged more or 

 less concentrically or more or less 

 radially, or may have no par- 

 ticular arrangement. Sometimes 

 these inner structures may be- 

 come almost or quite obsolete, so 

 that the joint faces have almost 

 the appearance of a typical synarthry; again, they may be very prominent with a 

 generally radial arrangement, so that except for the broad concentric outer border 

 the joint face resembles that of a typical syzygy. 



Carpenter described the syzygy between the elements of the IBr series in 

 Comaster novaigwneas and states that while in all the recent comatulids the 



^fO^r 



FIG. 223. LATERAL VIEW OF TYPE SPECIMEN OP THALASSOMETBA 

 HAWAIIENSIS. 



