100 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



Radial faces. — See Radial articular faces. 



Radial mouth.^ln those species of the Comnstericlae in which the mouth is excentric 

 or marginal it is situated either at the base of the anterior ray, or between the 

 bases of the anterior and right anterior rays; in the first case it is known as 

 a radial mouth, and in the second as an interradinl mouth (see fig. 25, and 

 compare with figs. 26-28, p. 69). 



Radial pentagon. — The more or less pentagonal ring formeil by the five radials, 

 mutually adherent, after the removal of all other structures (see figs. 441-443, 

 p. 351, which represent two-fifths of a radial pentagon). 



Radial 7-adials. — In the genus Promachocrinus, the radials which occupy the normal 

 radial position, in contrast to the interradial radials, wliich are situated in the 

 iiitorradial angles over the ends of the basal rays (see figs. 505, p. 371, and 549, 

 pi. 5, and pp. 191-194). 



Radial ridges. — On the centrodorsal; the ridges sometimes developed in the mid- 

 radial portion of the lateral surface (see figs. 9, p. 65, 227, p. 245, and pp. 230-232). 



Radial skeleton. — The Appendicular skeleton plus the Radials. 



Radial structures. — (1) Structures associated with the radials. 

 (2) Structures radially situated. 



Radially situated. — See Radial. 



Radials (RR). — The five plates from which the arms arise. These are in tlie same 

 line as the infrabasals and alternate in position with the basals and orals. The 

 radials are the most important plates in the crinoid calyx; they are always 

 present and undergo comparatively little change of form; in the comatulids 

 their size is reduced to a minimum (see figs. 2, p. 61, 3, p. 62, 9-12, 14, p. 65, 

 30, p. 71, 433-446, p. 351, and pp. 348-382). 



In two genera, Promachocrinus and Thaumatocrinus, there are 10 

 radials, 5 in the usual position, and 5 interradial situated in line with the 

 basals and orals; the former are the 5 radials of the other genera, while the 

 latter are secondarily derived from intcrradials. 



The radials are the equivalent of the termmals of the asteroids, and of the 

 ambulacrals bordering the jieristome m the urchins. 



Radianal (RA). — A plate occurring in the pentacrinoid young of the comatulids sit- 

 uated between the two posterior radials, usually more or less accommodated in a 

 concavity m the radial to the right of the posterior interradius, and resting 

 on the posterior basal, usually to the right of the median line, at the base of the 

 anal tube; in most developmg comatulids it is the only prominent interradial 

 plate; it is always resorbed early hi post-embr\-onic life (see figs. 413, p. 317, 

 553, pi. 5, 560-562, pi. 6, 576, pi. 9, 588, pi. 13, and 594, pi. 16, and pp. 331-335). 

 Heretofore tliis plate has always been mcorrcctly caUed the anal, under 

 the supposition that it represented the anal x of fossil forms.. 



The normal position of the radianal, in which it occure in most of the 

 fossil t}-pi>s in which it is developed, is beneath the right posteiior radial, 

 between that radial and the iufrabasal; it is the last remnant of a circlet of 

 five subradial plates. 



