4 i8 



PELMATOZOA. 



dorsal side of the calyx which is turned downwards, while the 

 ventral or oral side is turned upwards. The dorsal or inferior side 

 of the calyx is entirely enclosed in a series of polygonal calcare- 

 ous plates, which are more or less closely joined together. In some 

 cases the plates are united by firm sutures ; 

 while in others the plates are more or less 

 movably articulated ; and in still other 

 cases certain of the plates may be con- 

 nected by the peculiar mode of immovable 

 union, which has been already described 

 under the name of "syzygy." In rare 

 cases, as in the Devonian genus Hystri- 

 crinus, the calycine plates are furnished 

 externally with movably articulated spines, 

 resembling the prickles of the Echinoids. 



The following is the general arrange- 

 ment of the calycine plates in the Stalked 

 Crinoids (figs. 291 and 292). Resting 

 directly upon the summit of the highest 

 joint of the column is the cup-shaped basal 

 portion of the calyx, which is known as 

 the "basis," and which may consist of a 

 single or double row of plates. In those 

 Crinoids in which the basis is composed 

 of a single row of plates only — hence 

 termed "monocyclic" Crinoids — its com- 

 ponent plates rest directly upon the top-joint of the column, and are 

 known as the " basals." The " basals " are interradial in position, 

 and are homologous with the "genital plates" in the apical disc of 

 the Echinoids. The basals are usually five in number, but may be 

 reduced to four, three, or two ; and they may be invisible externally 

 owing to their concealment by the radials (as in most Comatulcz). 

 In the so-called "dicyclic" Crinoids, on the other hand, the true 

 " basals " retain their constant interradial position, but are separated 

 from the top-joint of the column by an intercalated row of plates 

 termed the " underbasals " (figs. 291 and 292, b), the basis thus 

 consisting of two successive cycles of plates. 1 The underbasals are 

 radial in position, and have no representative in the calyx of the 

 "monocyclic" Crinoids. It is not clear that they possess any 

 homologues in the apical disc of the Sea-urchins, as do the basals 

 and radials of the Crinoidal calyx. They were formerly compared 



291. — Side-view of the 

 calyx of Poteriocrinus multiplex, 

 from the Carboniferous Limestone 

 of Russia, of the natural size, 

 showing the top -joints of the 

 column (c) and the bases of the 

 arms, b, The cycle of "under- 

 basals"; /, The "basals"; r, 

 The "primary radials"; br, The 

 "second radials " (or first " brach- 

 ials" of some authors). (After 

 Zittel.) 



1 In the nomenclature of some writers, the plates of the dicyclic calyx are known 

 respectively as the "basals" and "parabasals" ; but the nomenclature of Dr P. 

 Herbert Carpenter, employed above, undoubtedly expresses the true homologies 

 of the monocyclic and dicyclic forms. 



