. Bulletin - United States National Museum. Science. 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, fi
. Bulletin - United States National Museum. Science. 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 tr
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