TERMINOLOGY 21 



when longitudinally divided are interradial, and the vertical sutures and cirri radial ; the outer 

 angles of the stem, when pentagonal, are usually interradial; the sides of the stem and the 

 outer angles of the lumen for the axial canal radial — the last three cases being subject to 

 exceptions due to secondary growth. In a monocyclic crinoid these positions are reversed. 

 By attention to these relations it is possible from the position of the cirri, and frequently from 

 that of the angles of the stem or axial canal, to determine whether a specimen is dicyclic or not, 

 though the infrabasals cannot be seen. There are no angular stems among the Flexibilia ; and 

 all of them are dicyclic. 



Radials (RR). The circlet formed by the first plate in each of the rays, or, the radially 

 situated circlet of plates above the basals, and this ring only. In some of the earlier crinoids 

 one or more of the radials appear as if transversely bisected, the lower segment of the com- 

 pound plate being called the inferradial, and the upper segment the superradial. A ray, or 

 radius, is the radial plus all the structures which it bears ; an interray, or interradius , means 

 all structures between the rays ; these terms are usually employed for structures considered 

 from the dorsal side, but properly speaking they include both dorsal and ventral structures. 



Brachials (Br). All plates beyond the radials in radial succession to the ends of the 

 arms. They are called fixed brachials in so far as they take part in the calyx; free brachials, 

 or arm-plates, when they do not. The brachials forming the first circlet above the radials, 

 whether fixed or free (formerly "costals"), are called primibrachs (IBr) ; those of the 

 second order (formerly " distichals "), secundibrachs (IIBr) ; those of the third order 

 (formerly " palmars "), tertibrachs (IIIBr) : and so on for the succeeding orders of brachials 

 to which formerly the name " post-palmars " was applied. A particular brachial in any series 

 is denoted by placing a small Arabic numeral after the symbol — IBr 2 , IVBr c . The axillary 

 brachials may be designated IAx, IIAx, etc. 



Ambulacrals (Amb). The rows of small plates enclosing the food-groove in the tegmen 

 and arms. They consist of adambulacrals or side-pieces, and covering-plates; the former 

 when present constitute the outer, the latter the inner, rows of plates. The covering plates 

 form a roof over the food grooves ; they are generally represented by two alternating rows of 

 small plates, more or less regular in their arrangement, which are movable upon the arms and 

 pinnules, but upon the disk only in those crinoids in which the mouth is exposed. 



Interradials (iR). All plates occupying the spaces between the rays proper, whether 

 they belong to the dorsal cup or to the tegmen. Interbrachials (iBr). Interradials in the 

 dorsal cup which are interposed between the brachials of the several orders. Plates between 

 brachials of the second and higher orders are inter secundibrachs, etc. (illBr, etc.) ; those of 

 the tegmen which lie between the ambulacra are interambulacrals (iAmb). The term " inter- 

 radials " includes all interbrachials, but the term " interbrachials " does not include those 

 interradials which lie within the radial circlet ; the latter plates, however, are not normally 

 represented in the Flexibilia, so that so far as this group is concerned the two terms are 

 synonymous. 



Radianal (RA). A plate disturbing the bilateral symmetry of the cup, located primi- 

 tively directly below the right posterior radial (being an inferradial restricted to the right 

 posterior ray), and in later genera obliquely to the left of it, either at the lower or the upper 

 corner. 



Anals. Interradials of the posterior side, forming the base of the anal structures. The 

 special or first anal plate (now usually designated .r) when present usually rests upon the 

 truncated upper face of the posterior basal and between the radials. Higher anal plates may 

 be present, even when the special anal plate is wanting ; they are interposed between the inter- 

 radials, following the median line of the posterior area, and may extend upward along the 

 tegmen where they form the backing or support for a conical tube. The first plate of the 

 tube may in some cases represent the radianal. 



