MONOGRAPH OF THE EXISTING CRINOIDS. 31 



against the outer border of the articular face, one side nearly coinciding with the 

 proximal border of the transverse ridge, but slightly inclined upward, and the 

 other running diagonally from the outer upper corner of the transverse ridge to 

 a point just below the center of the side of the central canal, where it forms with 

 the opposite side a sharp angle. In some cases these fossae take the form of 

 narrow isosceles triangles, the bases coinciding with the ends of the transverse ridge 

 and the apices lying near the center of the sides of the central canal. 



This pair of fossae has not previously been described in any comatulid, nor 

 does it appear to exist in any genus other than Pontiometra. It is probable from 

 their structure and situation that these fossse lodge an additional pair of inter- 

 articular ligaments. They may be known as fulcral ligament fossae from their 

 position on the fulcral line of the joint face. Their appearance is possibly a step 

 in the direction of the disintegration of the joint face as a muscular articulation 

 and the formation therefrom of a pseudosj r zygy such as is seen between the 

 elements of the IBr series in certain species of Comatula and Comaster. 



The dorsal ligament fossa is large, nearly semicircular in outline, about twice 

 as broad as long. The ventral border is not straight but forms a very broadly 

 obtuse angle, of which the apex is just below the central canal. 



The interarticular ligament fossse are equal in height to the dorsal ligament 

 fossa; their edges are straight and converge slightly but turn outward rather 

 abruptly at the distal outer corners. The distal border is somewhat convex, 

 straight, or slanting slightly inward and downward. The inner distal angles are 

 broadly rounded off, and the fossae are separated interiorly by a U-shaped gap 

 with parallel sides, which is about equal in width to the transverse diameter of 

 the central canal. This gap contains a shallow groove with a more or less bluntly 

 carinate ridge in the center. 



Muscular fossae are entirely absent. 



The space between the radials ventrally is filled with a spongy calcareous 

 deposit with a roughly radial arrangement which is slightly depressed in the 

 center and pierced by a small central channel. 



Diameter of radial pentagon at base. 7.5 mm. ; height of articular face (meas- 

 ured along the inclination), 3.1 mm.; width at transverse ridge, 3 mm.; nearest 

 approximation of adjacent articular faces. 0.8 mm. ; distance from center of rosette 

 to middle of outer dorsal edge of radial, 4 mm.; distance from center of ventral 

 surface of radial pentagon to middle of ventral outer edge of radial, 2.5 mm. : to 

 apex of interradial angle, 3 mm. 



MARIAMETRA SUBCARINATA. 

 Figs. 47, 48, p. 26. 



The interarticular ligament and muscular fossse slope regularly and evenly 

 inward from the transverse ridges, their planes making an angle of about 30 

 with the dorsoventral axis of the animal, and the plane touching the rim of the 

 dorsal ligament fossa has only a very slightly greater inclination. 



