ECHINODERMA TA—CRINOIDEA . 



489 



The calyx is composed of the 

 dorsal cup and the ventral disk 

 or tegmen, the arm regions form- 

 ing the Hne of demarkation be- 

 tween them (Fig. 1805, a). The 

 dorsal cup conforms in general 

 to the apical or abactinal system 

 of other echinoderms, the ventral 

 disk to the oral or actinal. 



The dorsal cup is composed of 

 a number of plates which have 

 a definite arrangement in horizon- 

 tally disposed series (Fig. 1804). 

 The radials (R) are the five plates 

 from which the rays (the arms 

 or arm trunks) may be traced; 

 their upper faces unite with the 

 brachials by straight, crescent- 

 shaped, or angular facets. In some 

 of the earlier crinoids one or more 

 of the R are compound, i. e., bi- 

 sected transversely, in which case 

 the two parts are distinguished as 

 superradials and inferradials. Be- 

 low the R and alternating with 

 them in position are the basals 

 (B) varying in number from two 

 to five. If only this single row 

 of the B intervenes between the 

 R and the column, the base is 

 monocyclic ; when there is a second 

 series of plates in the base below 

 the B and alternating with them 

 they are the infrabasals (ib.) and 

 the base is called dicyclic. All 

 plates above the R, in radial suc- 

 cession and constituting the arms 



Fig. 1803. Eucalyptocrinus crassus, with 

 stem. (After Hall.) 



