MONOGRAPH OF THE EXISTING CKINOIDS. 61 



proximally and with but slightly diverging sides, which is in depth equal to from 

 one-third to the entire length of the intermuscular furrow. 



There is no deposit of calcareous matter within the ventral cavity of the 

 radial pentagon. 



ZENOMETRA COLUMNARIS. 

 Figs. 93, 94, p. 02. 



The dorsal ligament fossa is about three times as broad as high, with the 

 proximal border moderately convex, becoming more strongly curved at the ends. 

 The ligament pit is very large and narrowly reniform, but shallow and ill defined. 



The transverse ridges join across the interradial angles. The lateral ends of 

 the dorsal ligament fossae are slightly confluent; the lateral edges of the inter- 

 articular ligament fossae are separated by a narrow but deep furrow, which is 

 bridged over distally by the lateral expansion of the muscular fossae. 



The interarticular ligament fossae are in shape almost equilateral triangles, the 

 inner ends of which are truncated by the broad high rim about the central canal. 

 Their height is equal to the height of the dorsal ligament fossa. 



The central canal is large and slightly oval. It is surrounded by a raised 

 area, which at the sides takes the form of a high narrow rim continuous with the 

 distal portion of the transverse ridge ; the central canal thus appears to be sunken 

 for about half its height below the distal edge of the transverse ridge. Distal to 

 the central canal this border becomes wider, and may equal in width the diameter 

 of the central canal. From it run the distal edges of the interarticular ligament 

 fossae in the shape of high narrow septa, gradually decreasing in height and finally 

 turning rather abruptly downward; between these lies the median intermuscular 

 septum, which at first decreases rather rapidly in height, then more slowly, con- 

 tinuing as a prominent ridge to the distal intermuscular notch. 



The muscular fossae are large, but comparatively narrow. Their inner (ap- 

 posed) edges are about twice as long as their lower edges, which border the inter- 

 articular ligament fossae. They are roughly triangular in form, with the outer 

 edges well rounded and projecting well beyond the outer edges of the interarticular 

 ligament fossae. The distal inner angles are much more broadly rounded than the 

 lateral angles. 



Distal to the transverse ridge the two lateral halves of each joint face make 

 an angle of 90 with each other, so that the inner part of the interarticular liga- 

 ment and muscular fossae is very deeply excavated. 



There is no deposit of calcareous matter on the inner portions of the radials. 



Height of the articular face (measured along the inclination), 3.4 mm.; width, 

 at transverse ridge, 2.6 mm. 



PSATHTROMETRA FRAGIL1S. 



Figs. 01, 92, p. 62. 



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

 long, well rounded proximally. The ligament pit is wider than the central canal, 

 but very narrow and but slightly marked. 



