MONOGRAPH OF THE EXISTING CKINOIDS. 57 



angle of about 60 with the transverse ridge, this angle being somewhat less in 

 small specimens. 



The inner edges of the ridges forming the distal borders of the interarticular 

 ligament fossae are swollen and stand out as rounded bosses. 



The intermuscular furrow is shallow and about half as broad as the central 

 canal. 



The muscular fossae are but little larger than the interarticular ligament 

 fossae and are approximately rhombic in shape, with the outer and inner distal 

 angles well rounded. Their outer edge is less than that of the interarticular 

 ligament fossae. 



There is more or less of a calcareous deposit within the ventral concavity of 

 the radial pentagon, so that the rosette is not visible in ventral view. 



Diameter of radial pentagon at base. 2.3 mm.; height of articular face (meas- 

 ured along the inclination), 1.4 mm.; width, at transverse ridge, 1.8 mm.; diameter 

 of ventral interarticular space, 1.8 mm. 



ERTTHROMETRA RUBER. 



Figs. 87, 88, p. 53. 



The dorsal ligament fossa is about two and one-half times as broad as high, 

 well rounded proximally. The ligament pit is about the size and shape of the 

 central canal, but is only faintly marked. 



The adjacent articular faces are separated interradially by a narrow shallow 

 furrow, which is about half as wide as the central canal. The sides of this 

 furrow are parallel, or it may be more or less constricted between the muscular 

 fossae. Proximally it is not constricted at the transverse ridge but gradually 

 bends outward, the edges continuing into the proximal borders of the dorsal 

 ligament fossae. 



The outer edge of the interarticular ligament fossae is slightly convex and 

 is continued, as in Hathrometra prolixa, into the outer edge of the muscular 

 fossae. It makes a general angle of about 60 with the median axis of the joint 

 face. The distal edge makes an angle of about 45 with the transverse ridge; 

 it consists of a low ridge which rapidly decreases in height, so that in its outer 

 fourth or fifth it is barely indicated, or is absent altogether. It takes its origin 

 relatively low down on the sides of the rim surrounding the central canal so that a 

 line parallel to the transverse ridge passing through the distal border of the canal 

 also passes through the inner bases of the distal borders of the interarticular 

 ligament fossae. 



The muscular fossae are small, being only slightly larger than the interarticular 

 ligament fossae. Their distal edges are about as long as, and are parallel with, 

 the distal edges of the interarticular ligament fossae, making, therefore, an angle 

 of about 90 with each other: outwardly they sweep in a broad curve and are 

 continued downward into the outer edges of the interarticular ligament fossae. 



The muscular fossae are separated interiorly by a broad, low intermuscular 

 ridge. 



