166 



BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



While in the comatulids the radiating furrows and ridges are always very 

 distinct, in the closely allied pentacrinites they do not occur, and at most the joint 

 face is indistinctly scalloped or beaded about the periphery. In the pentacrinites 

 also the surface of the joint face, which in the comatulids is perfectly flat, is 

 undulating. 



The joint faces of the syzygies in Atopocrinm siboc/cv are so extraordinary as 

 to deserve special notice. 



Laterally and dorsally the central canal is bounded by a high ridge of moderate 



width ; from this ridge there extends to 

 the dorsal margin of the joint face in the 

 dorsoventral line another ridge which at 

 first is about as broad as the ridge from 

 which it springs, but outwardly grad- 

 ually broadens slightly ; two similar 

 ridges extend outward, one from either 

 end of the laterodorsal ridge about the 

 central canal, making with the median 

 ridge an angle of about 45, or about a 

 right angle with each other. Just beyond 

 these lateral ridges, entirely unconnected 

 with the ridge about the central canal, 

 separated from the lateral ridges by a 

 ligament space about the same in shape 

 and size as the two lateral ridges, are two 

 more ridges, slightly broader than the 

 others; each has a very narrow fan-shaped 

 space beyond it. Beyond these two liga- 

 ment spaces, the distal borders of which 

 are approximately at right angles to the 

 dorsoventral axis of the joint face, are 

 two large obsolete muscular fossse, which 

 are about as high as the distance between 

 their proximal border and the dorsal 

 edge of the joint face; these are rounded- 

 FIO. 219.-LAIER.U, VIEW OK SPECIMEN OF s T *Lo- tr i an ,r u i ar i n s h a pe ; inwardly each rises 



METRA SPINIFEBA. > . 



somewhat, forming two parallel, very 



inconspicuous, low, and well rounded ridges, which are interiorly separated by a 

 shallow rounded groove which becomes more accentuated just beyond the central 

 canal, where it separates the inner ends of the inner pair of ridges; the inner ends 

 of the ligament spaces just beyond these ridges are bounded by the ridges border- 

 ing the muscular fossae interiorly. There is a deep intermuscular notch the sides 

 of which make an angle of from 60 to 90 with each other. The ridges on the 

 syzygial faces are high, and consequently the ligament fibers are long, appearing in 

 dorsal view almost or quite as long as those of the neighboring dorsal ligaments. 



