36 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



decrease very sli^^litly in width inwardly. In OU(jomefra the muscular fossa? are 

 very small, rounded trianjiular, with the distal borders convex and approximately 

 parallel to the transverse ridge; the maximum length is about one-half the breadth. 

 In Colohometra and in Cyllometra they are trapezoidal with the acute angle down- 

 ward and inward ; in all of the genera except Cyllometra they are smaller than the 

 interarticular ligament fossa;. 



The ventral surface of the radial pentagon is pentagonal or very bluntly stel- 

 late in outline Except in Cenometra the ventral plug is very poorly developed 

 and usually more or less confined to the central portion of the radial pentagon. 

 There may or may not be a free central canal. 



CENOMETRA UNICORNIS. 

 Fig. .53, p. 33. 



The dorsal ligament fossa is broad and well rounded, slightly over twice as 

 broad as high: the proximal border is strongly and very evenly curved. The 

 ligament pit is considerably smaller than the central canal, and is circular; the 

 central canal is moderate in size and slightly oval. 



The transverse ridges of adjacent joint faces are slightly separated from each 

 other; the dorsal ligament fossie are widely separated; the elements distal to the 

 transverse ridge are separated interradially by a shallow furrow, at first about as 

 wide as the intermu.scular furrow, but distally becoming slightly wider. 



The interarticular ligament fossas are large, their distal borders being about 

 as high above the transverse ridge as one-half the length of the ridge. The distal 

 and the inner edges of the interarticular ligament fossie are of about the same 

 length; the inner edges are parallel and straight, and are separated by a rather 

 deep intermuscular furrow, which is about as broad as the ligament pit. The distal 

 edges are straight and parallel to the transverse ridge, or are slightly convex, the 

 chord of the convexity being parallel to the transverse ridge; the inner distal 

 angles are more broadly rounded than the outer; the outer borders are nearly 

 straight. 



The muscular fossa? are narrow, being usually about as broad as the longi- 

 tudinal (shorter) diameter of the central canal. Their distal boi-ders are parallel 

 to those of the interarticular ligament fossfe, but are usually somewhat more con- 

 vex. The inner distal angles are broadly rounded, so that the intermuscular 

 furrow is not appreciably constricted; the outer borders of the muscular fossffi 

 usually do not project beyond the line of the outer borders of the interarticular 

 ligament fossa;. 



The interior of the radial pentagon is occupied by a calcareous deposit which 

 rises to the height of the distal edge of the muscular fossas: it is irregularly radially 

 striate on the surface and is slightly depressed centrally in the region of the mod- 

 erately developed funnel-shaped central canal. 



Diameter of radial pentagon at base, 5.4 mm.; height of articular face 

 (measured along the inclination), 2.8 mm.; width, at transverse rido-e, 3 mm.; dis- 



