38 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



radial furrow which is about as broad as the longer (horizontal) diameter of the 

 central canal. 



The outer borders of the interarticular ligament fossae make an angle of about 

 60 with the transverse ridge, and are straight or slightly concave. The distal edges 

 run from the outer distal angles toward a point on the transverse ridge halfway 

 between the central canal and the end until near the central canal, when they turn 

 downward somewhat, the inner edges of the two fossae together forming a broad 

 rounded Y just over the central canal. 



The muscular fossae are triangular, the apices at the outer distal angles of the 

 interarticular ligament fossae. The distal borders are horizontal, or slope inward 

 very slightly; the inner distal angles are rather broadly rounded and the inner 

 sides are parallel, separated by a shallow intermuscular furrow, which is about as 

 broad as the ligament pit in the dorsal ligament fossa. 



The central plug is represented by a loose calcareous meshwork in the center 

 of the ventral surface of the radial pentagon which conceals the rosette. 



Diameter of radial pentagon at base, 2 mm.; height of articular face (measured 

 along the inclination), 0.8 mm.; width, at transverse ridge, 1.4 mm.; distance from 

 center of rosette to middle of dorsal outer edge of radial, 0.8 mm. ; distance from 

 center of ventral face of radial pentagon to middle of ventral border of radial, 0.6 

 mm. ; to apex of interradial angle, 0.9 mm. 



TROPIOMETRID^B. 



The dorsal ligament fossa is semielliptical, four or five times as broad as high. 

 The ligament pit is slightly marked, but large ; in shape it is transversely elongate. 



The distal borders of the interarticular ligament fossae are parallel to the 

 transverse ridge, though they may curve downward somewhat inwardly. 



The muscular fossse are large and well rounded, about twice as high as the 

 interarticular ligament fossae. Their lower inner corners are occupied by large sup- 

 plementary muscle plates. 



The ventral surface of the radial pentagon is devoid of calcareous deposits. 



TROPIOMETRA MACRODISCUS. 

 Fig. 64, p. 33. 



The radial articular faces of Tropiometra macrodiscus do not differ greatly 

 from those of T. encrinus, as described below. The dorsal ligament fossa is nar- 

 rower, with more pointed ends ; it is about five times as broad as high ; the central 

 canal is more nearly circular ; the distal borders of the interarticular ligament fossae 

 run in a straight line from the distal angles to the proximal end of the intermus- 

 cular furrow, and in the angle between these rather large second supplementary 

 muscle plates are developed. The first supplementary muscle plates are larger than 

 those of T. encrinus, occupying about half of the area of the muscular fossae instead 

 of one-fourth or one-third. The muscular fossae as a whole are proportionately 

 slightly smaller and are rather less rounded. They are in general proportionately 



