54 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



The height of the muscular fossae is about equal to that of the interarticular 

 ligament fossae. Their inner and proximal edges are about equal in length and 

 make an angle of about 45 with each other. The outer and distal edges also are 

 of equal length, but shorter than the others, so that the outer edges continue the 

 direction of the outer edges of the interarticular ligament fossae. 



The muscular fossa? are separated interiorly by a broad and rather high inter- 

 muscular ridge. The intermuscular notch is small and shallow, or nearly obsolete. 



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



Diameter of radial pentagon at base, 1.5 mm. ; height of articular face (meas- 

 ured along the inclination), 0.8 mm.; width, at transverse ridge, 0.7 mm.; distance 

 from center of rosette to middle of outer distal edge of radial, 0.6 mm. 



THTSANOMTKTRINJE. 



THYSANOMETRA TENEULOIDES. 

 Figs. 89, 90, p. 62. 



The dorsal ligament fossa is very narrow, four times as broad as high, or even 

 somewhat broader; the proximal border is regularly semielliptical. The ligament 

 pit is about as large as the transversely oval central canal, but is only faintly 

 marked. 



The transverse ridges are nearly or quite confluent across the interradial planes, 

 but the dorsal ligament fossae are widely free ; the interarticular ligament fossae are 

 separated interradially by a moderately wide deep furrow. The muscular fossae 

 are usually in close lateral apposition, but they may be separated in a manner 

 similar to the interarticular ligament fossae. 



The outer edge of the interarticular ligament fossae is usually more or less 

 strongly concave, and makes typically a general angle of about 60 with the trans- 

 verse ridge. It is about half as long as the distance between the outer ends of the 

 transverse ridge and the outer edge of the central canal, or slightly shorter. The 

 distal edge runs inward and upward at an angle of nearly or quite 30 to the trans- 

 verse ridge, soon beginning to curve and making a broad sweep to a point on the 

 intermuscular structures slightly farther from the transverse ridge than the distal 

 lateral angles of the interarticular ligament fossae. The height of the convexity is 

 rather over one-third of the distance along the septum from the inner to the 

 outer end. 



Beyond the central canal, separated from it by a narrow ridge, there is a more 

 or less rhombic area, depressed centrally, the proximal sides of which are delimited 

 by the converging inner ends of the distal borders of the interarticular ligament 

 fossae, which stand up as low, narrow, rounded ridges above the general surface. 

 The maximum width of this area, which is at the point where the distal borders 

 of the interarticular ligament fossae in curving outward become free, is about equal 

 to the transverse (greater) diameter of the central canal. From this point onward 

 it gradually narrows anteriorly and comes almost to a point at the proximal end 

 of the intermuscular notch. 



