MOKOGRAPH Ol" THE EXISTING CKINOIDS. 



163 



In the proximal jiortion of the postradial series in certain large comatulids, 

 especially in the genera Co-ma,<iter and Comatvla, there is a well-marked tendency 

 for the synarthries to lose their normal joint face sculpture and either to become 

 nearly plane through the almost complete suppression of the median dorsoventral 

 ridge (part 1, fig. 36, p. 75) or to acquire irregularly meandering or concentric 

 ridges similar to those seen be- 

 tween the plates in many of the jf^ 

 Flexibilia (part 1, figs. 37. .38. 

 p. 75). 



In the articulations between 

 the cirri and the centrodorsal the 

 articular surfaces are plane, or. 

 very rarely, scalloped at the pe- 

 riphery. Only in the Atele- 

 crinidfe is a transverse ridge with 

 deep ligament fossae on either 

 side developed. 



In the ontogeny of the coma- 

 tulids. while the characteristics of 

 the brachial synarthries are evi- 

 dent from the first appearance of 

 the ossicles on either side, the 

 articulations between brachials 

 ultimately to become united by 

 syzygy do not begin to develop 

 the radiating ridges imtil those 

 brachials have acquired most of 

 the features of the fully formed 

 arm segments. 



Since the syzygial joint faces 

 are smooth and unmodified 

 throughout life in the penta- 

 crinites. and in the comatulids 

 do not develop radiating ridges 

 until a verv late stage either in 



. " "- . I"iG. 217. — Lateral vtew of spkcimen of Parametra oriox. 



the growing young or m regener- 

 ating arms, we appear to be justified in assuming that the brachial syzygies are 

 fundamentally unions of smooth surfaces comparable to the unions between the 

 calyx plates or radials which, as the latter do frequently in many types, in the 

 comatulids have developed ridges that instead of remaining irregular and meander- 

 ing have become straight and regularly radiating to meet the equal stresses exerted 

 in all the radii on a circular joint face. 



The occasional transformation of the synarthries in Comaster and Gorruitula 

 into articulations indistingui.shable from true syzygies (part 1, figs. 37--iO, p. 7-')), 



