40 BULLETIN 82, UNITED STATES NATIONAL MUSEUM 



as Capillaster sentosa, have an articulation between the elements of the IBr series 

 that approaches a cryptosynarthry. Capillaster may sometimes have only a single 

 synarthry in each arm between the first two brachials. Comatula and Comatulella (?) 

 alone lack synarthries. Synarthrial tubercles and synarthrial backward projections 

 are lacking. 



Zygometridae. The articulation between the elements of the IBr series is de- 

 veloped as a syzygy. Normal synarthries always appear between the two ossicles 

 following each axillary. There are scarcely any synarthrial tubercles. 



Himerometridae.- Gisle'n did not examine any of the synarthries in the IBr series, 

 but he found that the following synarthries are typical. The synarthrial tubercles 

 are often knoblike and projecting. There are no synarthrial backward projections. 



Mariametridae.in Stephanometra the synarthry in the IBr series shows a pair 

 of deep and stout pits on either side of a fairly narrow dorsoventral beam. About 

 the periphery is a broad contact margin. The capacity for movement is extremely 

 slight. The more distal synarthries are typical. There are no synarthrial tubercles. 

 The synarthry in the IBr series in Lamprometra palmata corresponds in a high degree 

 to that in Stephanometra. The more distal synarthries are high and laterally com- 

 pressed, as in all forms with numerous arms. 



Colobometridae. In Pontiometra andersoni the synarthry in the IBr series cor- 

 responds to that in Stephanometra. The more distal synarthries are high and laterally 

 compressed, more so than in other multibrachiate types. There are indistinct syn- 

 arthrial tubercles. In having rhomboid al joint faces Oligometrides represents a unique 

 type. The synarthries in the other forms offer very little of interest. Synarthrial 

 backward projections are absent or, if present, weak. Gisle'n studied the synarthries 

 in the IBr series in Austrometra, Cenometra, and Cyllometra. 



Tropiometridae. The synarthrial faces of the articulation between the elements 

 of the IBr series in Tropiometra ajra macrodiscus remind one very much of the same 

 faces in the large species of Comasteridae. The joint pits are scarcely marked plainly, 

 are bounded indistinctly, and are rather shallow. The capacity for movement is 

 slight. In the synarthries between the first two brachials the articular pits are not 

 developed in a semilunar shape, but only the dorsal half of each half moon is found 

 marked. Both the fossae, however, are well circumscribed here. Synarthrial tuber- 

 cles are absent or indistinct. 



Calometridae. The radials are often produced anteriorly in the interradial angles 

 between the IBr 1( and possibly as a result of this the first synarthrial face shows large 

 wing-shaped appendages on both sides of the joint face. The union as seen from the 

 dorsal side is often very close, and the mobility therefore presumably very slight. 

 The fossae are, however, well marked. The more distal synarthries are of the usual 

 type. Usually the synarthrial tubercles are indistinct, but they are extraordinarily 

 large in Neometra diana. In Calometra discoidea there are synarthrial backward pro- 

 jections. 



Ptilometridae. There is never any synarthrial backward projection. The ca- 

 pacity for lateral motion is slighter than in the Thalassometridae. 



Thalassometridae. The contour of the synarthrial face is often somewhat angular, 

 owing to the lateral flattening or "wall-sidedness" of the post-radial series. The 

 synarthrial face is somewhat produced into an acute angle dorsally, where synarthrial 



