140 



ci'inoid consists of a dorsal cup or calyx, placed upon a f>t((Ik. by means of 

 ■which it is attached, and bears a fringe of amis, variously divided and 

 furnished with jointed appendages or ^M«?w/7es. The calyx is composed 

 of a number of plates, which have a definite arrangement, in horizontally 

 disposed series ( Fig. 25). The lowest of these are the baxuls, though in 

 many forms an additional series, the infrabasals, may underlie and alter- 

 nate with the basals. Ne.xt above the basals, and alternating with them 

 in position, are the radials, five in number, so called because they are in 

 line with the rays or arms. Heferring tlie position of the inferior plates 

 to that of the radials, we find that the basals are always situated inter- 

 rddidUy, while the infrabasals are situated radially. Above the radials 

 lie the brachials. These vary greatly in niuiiber and kind, sometimes 

 articulating directly with the radials, in which case all the brachials are 

 free, and sometimes having their lower series fixed and immovable, thus 

 forming a part of the calyx. The brachials lying directly upon the radials 

 are the costals ; of these there may be one or more series, when they a^e 

 distinguished from below up &s primary (cost.'), secondary (cost.'^), etc. 

 The uppermost costal of each ray is commonly axillary, i. e., pentagonal 

 in outline, with two upper joint edges inclined from each other. On 

 these rest the distichals, of which there are ten in each series. Secondary 

 distichals (dist.^) may rest upon the primary ones (dist.'), and may in 

 turn support the palmars, of which there would be twenty in a normal 

 series. Above these, on further division, are the post-2Mlmars, which are 

 often very numerous. Two types of arms can be distinguished, those 

 composed throughout of one series of plates ( uniserial), or those made up 

 of a double series (biserial), the plates of which usually interlock to a 

 greater or less extent. These latter are the more specialized, always 

 beginning uniserially. 



Between the radials are often found additional plates, the uitcrradials, 

 which may vary in number. 



Between the distichals of one ray may occur the interdistichals, which 

 are situated radially. Between the distichals of adjacent rays may occur 

 the interbracMals, and these will be situated interradially. An anal 

 interradius is present in unsymmetrical forms. The tegmen forms the 

 cover, or ventral part, of the calyx, and is composed of plates either 

 closely anchylosed, or held together by a leathery membrane. In the 

 Palseozoic Camerata the plates of the ventral disc fit closely and they are 

 considerably thickened, forming a very rigid, more or less convex, vault, 

 from which may arise the plated anal proboscis. 



The mouth of Paheozoic crinoids is usually beneath the tegmen, the 

 only external opening in the tegmen being that of the eccentric anus. 

 From the mouth, radiating grooves or canals commonly pass outward to 

 the arms, in which they are continued. These are the ambulacral 

 grooves, along which the food, caught on the arms, is conveyed to the 

 central mouth. These grooves ma}- be open or covered by plates. "Within 

 the cavity of the calyx are the viscera. 



The stalk, or stem, is composed of a varying number of joints, which 

 ore circular, elliptical, or angular in cross-section (Fig. 29). The joint 



