140 



crinoid consists of a dorsal cup or calyx, placed upon a stalk, by means of 

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

 furnished with jointed appendages or pinnules. The calyx is composed 

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

 disposed series (Fig. 25). The lowest of these arc the basals, though in 

 many forms an additional scries, the infrabasaU, may underlie and alter- 

 nate with the basals. Next above the basals, and alternating with them 

 in position, arc the radials, live in number, so called because they arc in 

 line with the rays or arms. Referring the position of the inferior plates 

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

 radially, while the infrabasals arc situated radially. Above the radials 

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

 articulating directly witli 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 are 

 distinguished from below up as primary (cost. 1 ), 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 disticlials, of which there arc ten in each series. Secondary 

 distichals (dist.-j may rest upon the primary ones (dist, 1 ), and may in 

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

 series. Above these, on further division, are the post-palmars, 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 uniserial 1 v. 



Between the radials are often found additional plates, the inten-adials, 

 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 interbrachials, and these will be situated interradially. An anal 

 interradius is present in unsymmetrical forms. The te'gmen 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 

 Palaeozoic 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 Palaeozoic crinoids is usually beneath the tegmen, the 

 ouly 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 ambulacra! 

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

 central mouth. These grooves may be open or covered by plates. Within 

 the cavity of the calyx are the viscera. 



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

 are circular, elliptical, or angular in cross-section < Fig. 29). The joint 



