372 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



or less prominent furrows, each lying over one of the sutural lines which mark the 

 limits of the individual radials (figs. 466, 468, p. 359). 



P. H. Carpenter noted that on the dorsal surface of the radial pentagon in 

 Antedon and in Leptometra the sides of these interradial furrows are simple and 

 straight; but in many of the other genera, including most of the Oligophreata and 

 many of the Macrophreata, that portion of the dorsal surface of each radial which 

 is next to its truncated lateral edge is raised into a sort of curved ridge or fold, so 

 that in the natural condition of mutual apposition of the five radials the dorsal 

 interradial furrows become somewhat lancet shaped (figs. 454, 457, 458, p. 355). 

 They correspond with the interradial grooves on the ventral surface of . the sub- 

 jacent centrodorsal (figs. 236, 241, 242, p. 249), and in the cavity formed by the 

 apposition of the edges of these grooves lie the five rays of the basal star (figs. 416- 

 427, p. 321). These interradial furrows on the dorsal surface of the radial pentagon, 

 like the interradial grooves on the ventral surface of the centrodorsal, are entirely 

 devoid of pigment, so that they commonly stand out sharply as five white leaflets 

 on a more or less yellow, reddish, dark brownish or purplish background. 



Each individual radial has the form of a somewhat irregular truncated pyramid 

 (figs. 433-446, p. 351). The dorsal surface is usually almost entirely or quite con- 

 cealed by the centrodorsal (figs. 431, 432, p. 349); it is nearly triangular in outline 

 (figs. 433-434, p. 351), the apex being inward, deviating from a true triangle in having 

 the outer side somewhat convex and the opposite apex more or less truncated. In 

 contour it may be nearly flat, but there is usually an approach to the form taken by 

 the surface of a cone; there is no curvature along the radial axis, but the tangential 

 planes parallel to the dorsoventral axis of the animal show from the outer edge of 

 the radial inward a convexity the radius of curvature of which becomes gradually 

 shorter as one nears the center of the animal, or the inner end of the radial. This 

 curvature is strongest in the interradial 'angles, decreasing toward the midradial 

 axis, often so rapidly that nearly the entire dorsal surface is practically flat. If a 

 part of the dorsal surface project beyond the rim of the centrodorsal, this external 

 portion commonly makes in the midradial axis an obtuse angle with the concealed 

 portion, and this angle occasionally approaches so near to 90 that in an external 

 view the radials appear to be standing vertically. 



The lateral faces by which the radials are in mutual contact are flat (figs. 437, 

 438, 442, 444, 446, p. 351, and 549, 551, 552, 554, 557, pi. 5), and approximate in 

 shape a right-angled triangle with a concave hypothnuse. The inner edge, forming 

 the boundary between the lateral and inner faces, is typically perpendicular to the 

 plane of the radial pentagon, but it is often more or less obscured by the develop- 

 ment of the central plug, to be later described ; the lower edge, between the lateral 

 and the dorsal surfaces, is usually cut away to accommodate the basal rays; the 

 outer edge is concave as a result of the sculpture incident to the development of the 

 articular facet. 



The inner ends of the radials are oblong in general outline, and of very vari- 

 able height (figs. 437, 438, 442, 444, 446, p. 351, and 549, 551, 557, pi. 5) ; the upper 

 edge is usually concave or more or less deeply incised or notched ; the general sur- 

 face is usually much obscured by the deposit of intercalicular calcareous rods and 

 lamina which, when abundant, form the so-called central plug (fig. 11, p. 65). 



