L\\ BULLETIN 82, UNITED STATES NATIONAL MUSEUM 



synostosis, specialized for particular purposes. One or the other form has developed 

 in response to the differenl demand made l>y the articulation in question. Some- 

 times, bowever, the syzygy seems to develop into the synarthry. In Crossometra 

 septmtrionalia, in which usually the first two brachials are united by syzygy, Gisl6n 

 observed in one ease two lateral concavities near the nerve lumen. The center of the 

 joint face therefore represents a synarthry, while the peripheral portion is of the normal 

 syzygyial type. Catoptometra (magnified minor) shows possibly a somewhat different 

 direction in it > de\ elopment. In fully grown individuals the articulation between the 

 IBr, and IBr.- is Byzygial; in the young this articulation is furnished about its periph- 

 ery with weak syzygial septa with a slightly stronger dorsoventral thickening which 

 gives the joint face a somewhat synarthrial aspect. 



typical syzygies scarcely occur in recent stalked crinoids. Calamocrinus and 

 Ptilocrinua have typical synostoses, as have a large number of pentacrinites also. 

 In the pentacrinites, however, there is an evident tendency toward the development 

 of Byzygies. Here, as in the fossil forms, there is found the first tendency toward 

 the formation of syzygies, foreshadowed by a weak fluting of the dorsolateral margin 

 of the joint face. The marginal ridges gradually extend toward the center, and when 

 they reach it the typical syzygy appears. The species of Mitacrinus have almost 

 typical synostoses, which, however, sometimes show a very indistinct marginal stria- 

 tion. Syzygies are indicated about as feebly in Annacrinus. On the other hand, a 

 syz)-gial type is rather distinct in Hypalocrinus naresianus and in Cenocrinus asteria. 



In describing the syzygies of the comatulids, Gislen called the thickened margin 

 of the nerve lumen the areola. The radiating ridges he called the septa. They are 

 complete when they reach the areola, or incomplete when they run only part way in 

 from the periphery of the joint face. They are called finished when they arise within 

 the margin of the joint face and continue to the center. They are forked when two 

 fuse a short distance within the margin. When the septa are both incomplete and 

 finished, there result small, low, more or less rounded protuberances — the tubercles. 

 In many of the larger forms the ventral septa especially show a tendency toward divi- 

 sion into rows of tubercles. Sometimes concentric ridges — synapticulae — run at cer- 

 tain distances from the nerve lumen. The areola is often connected with the ventral, 

 usually grooveless, part of the joint face by a more or less broad smooth line— a ven- 

 tral beam. Usually the median portion of this ventral beam is occupied by a pit 

 or furrow, so that there are two parallel ventral beams. These ventral beams may 

 be developed as coarse bow-shaped septa that run out to the margin of the joint face. 



These ridges and other elevations as found in different forms arc marked by dif- 

 ferent degrees of sculptural relief. Gislen denoted the strength of relief by figures 

 from 1 to 4. The Ggure 1 was assigned to forms having the septa only feebly indicated, 

 as for instance certain Charitometridac; syzygies in which the septa stand out in high 

 relief, such as those in Heliometra glacialis, are given the figure 4. 



The elevations on the distal face of the hypozyL_ r al are matched by corresponding 

 elevations on the proximal face of the epizygal. When the syzygial articulation is 

 observed from the dorsal side of the arms it is seen as a dotted line, the dots marking 

 the ends of the canals between the septa which are traversed by the ligaments. 



Gislen noted that the development from synostosis to syzygy evident in the phylo- 

 genetic history of the crinoids appears again in the ontogeny of the comatulids. In 



