18 BULLETIN 82, UNITED STATES NATIONAL, MUSEUM. 



Proximally the intermuscular furrow is about as broad as the central canal, 

 from which it is separated by a narrow rim, or it may be slightly broader. It 

 gradual!}' narrows distally, being scarcely half as broad between the muscular fossse. 



The muscular fossse are small and narrow, slightly crescentic, with the distal 

 edges almost parallel with the proximal. The inner angles are inserted within the 

 distal inner angles of the interarticular ligament fossse; they are about as broad as 

 the width of the intermuscular furrow between them. 



The inner portion of the radial pentagon is obscured by a deposit of calcareous 

 matter, the surface of which is marked with narrow radiating furrows. There is 

 no open central canal. This deposit, though entirely concealing the inner ends and 

 surfaces of the radials and obliterating the sutures between them, is thin, and 

 follows the contour of the inner surface of the radials, so that in a general view the 

 ventral surface of the radial pentagon appears deeply hollowed. 



Diameter of radial pentagon at base, 3 mm. ; height of articular face. 1 mm. ; 

 width, at transverse ridge, 2 mm.; distance from center of rosette to middle of 

 dorsal outer edge of radial, 1.7 mm. ; distance from center of ventral face of radial 

 pentagon to middle of ventral edge of radial, 0.9 mm.; to apex of interradial 

 angle, 1.3 mm. 



ZYGOMETRID^. 



The articular faces of the radials lean inward at an angle of approximately 

 45 to the dorsoventral axis. 



The excavation of the various elements of the joint face is moderate, as 

 there is but a limited amount of motion to be accommodated. 



The relatively deeper and more extensive excavation of the joint faces has 

 tended to narrow the broad flat area distal to the transverse ridge, as seen in 

 the Comasterida\ making of it a narrow rim about the central canal which slopes 

 downward on either side to the transverse ridge. 



The joint face elements on either side of the transverse ridge are in the same 

 plane, or, if not, the dorsal ligament fossa makes a smaller angle with the dorso- 

 ventral axis of the animal than the elements distal to the transverse ridge. 



The dorsal ligament fossa is as high as, or slightly higher than, the inter- 

 articular ligament fossae, and its proximal border is usually slightly flattened. 

 The ligament pit is strongly marked, broadly oval, larger than the central canal. 



The central canal is moderately large and broadly oval. 



The joint face elements distal to the transverse ridge are separated in the 

 median line by a rather narrow intermuscular furrow which does not exceed 

 one-third of the transverse diameter of the central canal in proximal width ; 

 distally its sides diverge slightly, and it is not constricted between the muscular 

 fossae. 



The interarticular ligament fossae are approximately triangular, with the 

 proximal inner angles broadly truncated by the elevation about the central canal. 



The muscular fossae are narrow and crescentic, their greatest diameter being 

 less than one-third of the height of the interarticular ligament fossa?. Their 

 inner ends rest upon the distal portion of the rim about the central canal: their 



