66 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



ligament fossae; in general they are rounded triangular, their outer edge con- 

 tinuing in the same direction as the outer edge of the interarticular ligament fossae. 

 The distal angle is strongly rounded, this involving usually the outer portion of 

 the lateral edge. The intermuscular notch is moderately deep, and rounded 

 proximally; its sides may be nearly parallel, or they may make an angle of nearly 

 90° -with each other. 



There is no calcareous deposit within the ventral surface of the radial pentagon. 



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

 ured along the inclination), 1.1 mm.; width, at transverse ridge, 1.2 mm.; distance 

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



THAUM.iTOMETRA TENinS. 

 Figs. 108-111, p. 67. 



The dorsal ligament fossa is narrow, about four times as broad as high, with 

 the proximal border regularly semielliptical, sometimes slightly flattened. The 

 ligament pit is very narrow transversely and faintly marked, but is broader than 

 the central canal. 



The ends of the transverse ridges are in contact in the interradial angles, but 

 the dorsal ligament fossae are widely free. The interarticular ligament fossae are 

 separated interradially by a moderately broad V-shaped space, the sides of the V 

 being slightly incurved, which is closed anteriorly by the apposition of the mus- 

 cular fossae. 



The high narrow ridge forming the distal border of the interarticular liga- 

 ment fossae at its inner end makes an angle of from 30° to 40° with the transverse 

 ridge. In young individuals, as in small specimens of Ilathrometra tenella, this 

 direction is maintained throughout its entire length, but in large individuals 

 it gradually curves about so that its outer half becomes parallel to the transverse 

 ridge, or even slopes slightly downward. In profile the crest of the ridge forming 

 the anterior border of the interarticular ligament fossae is seen to be at the inner 

 end strongly concave, from this curved portion running straight to the outer end, so 

 that the ridge gradually diminishes in height. Tlie height of the interarticular 

 ligament fossae is about equal to that of the dorsal ligament fossa. 



The central calcareous structures of the joint face resemble those found in 

 Florometra asperrima, but the triangular area is slightly smaller. 



The muscular fossae are large and well rounded, their height being somewhat 

 greater than that of the interarticular ligament and dorsal ligament fossae com- 

 bined. They are subtriangular, with the distal angles very broadly rounded, es- 

 pecially in the larger specimens. The intermuscular notch, which is moderately 

 broad and may be either acute or rounded proximally, extends inward for some- 

 what more than one-third the distance from the distal edge of the muscular fossae 

 to the central canal. 



There are only a few slender strands of calcareous deposit within the ventral 

 surface of the radial pentagon, so that the rosette is in plain view from the ventral 

 surface. 



