64 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



HATHROMETRA TENELLA. 

 Figs. 102-105, p. 67. 



The dorsal ligament fossa is narrow, from three to three and one-half times 

 as broad as high, with the proximal border regularly elliptical or slightly flattened. 

 The ligament pit is somewhat, usually considerably, broader than the central canal, 

 but narrow dorsoventrally. 



The transverse ridge is narrow, rising somewhat inwardly to meet the narrow 

 but very high rim about the central canal. The central canal is large and trans- 

 versely oval. The rim about the central canal is broader distally than laterally. 

 From the middle of the distal edge arises the intermuscular septum ; this at first 

 is as high as the rim and comparatively broad, but it rapidly decreases in height 

 and width for a short distance until it becomes a low, narrow septum, when it 

 rather abruptly assumes a very gradual decrease in height. It extends to the 

 intermuscular notch. On either side of the intermuscular septum, nearly over 

 the lateral borders of the central canal, arise the ridges forming the distal border 

 of the interarticular ligament fossae. These run outward, making an angle of 

 about 40 with the transverse ridge, and are practically straight; like the inter- 

 muscular septum they are at first high and comparatively broad, but gradually 

 become low and narrow : as a rule their decrease in height and width is very regu- 

 lar, there being no abrupt change as in the case of the intermuscular septum. 



The interarticular ligament fossoe are of the same height as the dorsal ligament 

 fossa. Their outer edges are strongly and regularly concave and their general 

 direction makes an angle of from 70 to 80 with the transverse ridge ; their distal 

 edges make an angle of about 40 with the transverse ridge ; their inner angles are 

 truncated. The interarticular ligament fossa; are separated interradially by a 

 narrow shallow furrow with parallel sides which is bridged over distally by the 

 muscular fossae, these being in close lateral contact. 



The muscular fossae are rounded triangular and very large; they are about twice 

 as high as the interarticular ligament fossre, or as high as the distance from the 

 proximal edge of the dorsal ligament fossa perpendicularly to the distal apex of 

 the interarticular ligament fossae. Their distal angles are more rounded than their 

 lateral angles, but are relatively sharp. Distally the muscular fossa3 are separated 

 interiorly by an intermuscular notch, which is usually about as deep as the dorso- 

 ventral (lesser) diameter of the central canal, narrow, and sharply pointed 

 proximally. It may, however, be closed by the approximation of the muscular 

 fossae on either side, or it may make a rather wide rounded V ; occasionally it may 

 extend considerably downward. 



The excavation of the joint face elements is deep, so that the halves on either 

 side of the dorsoventral plane distal to the transverse ridge make an angle of about 

 90 with each other. 



There is no calcareous deposit on the inner surface of the radials. 



Diameter of radial pentagon at base, 2.8 mm.; height of articular face (meas- 

 ured along the inclination), 1.9 mm.; width, at transverse ridge, 1.6 mm.; distance 



