vm ECHINODERM AT A MORPHOLOGY OF SKELETON 329 



(often even very small) part of it; other plates take part in its 

 structure, as we shall see more in detail when describing the peri- 

 somatic skeleton. The border of radials round the apical capsule 

 becomes more or less markedly disturbed by the appearance of 



FIG. 2S9. Apical system of Cyatho- 

 crinus. For lettering see p. 317. inn, Anal 

 interradial. 



FIG. 200. Marsupites ornatus. Plates of 

 the dorsal cup. For lettering see p. 317. 



special " anal plates " in the posterior unpaired interradius ; these 

 specialised anals occur very frequently in palaeozoic Crinoids (Fig. 291). 



The Crinoids with dieyelie base (with infrabasals, Figs. 289 

 and 290) are: (a) most Inadunata / (b) among the Camemta, the 

 families of the Reteocrinidw p. p., 

 Rhodocrinidce, Gh/ptasteridce, and 

 Crotalocrinidffi ; (c) the Articulata 

 (Ichthyocrinidw) ; (d) the Canalicu- 

 lata, in which, it is true, the infra- 

 basals are often either fused with 

 the uppermost joint of the stem 

 or atrophied, at least in the adult ; 

 such are conveniently termed 

 Pseudomonoeyelie. 



The Crinoids with monoeyelic 

 base (without infrabasals, Fig. 291) 

 are, apart from a few Inadunata, 

 the Camerate families of the 

 Melocrinidce, Actinocrinidce, Platy- 



_, -7 TT -, ., -7 FIG. 291. Actinocrinus proboscidalis. Plates 



cnmdce., Hexacnmdce, Acrocnmdce,, of the dorsal cup . Foi . p lflttering see P . 317. 

 Barrandeocrinidce, Emalyptocrinidce. 



Instead of the typical five infrabasals and five basals there are 

 very often found four, three, or even only two plates in these rings ; 

 this is especially the case in extinct Crinoids belonging to the orders 

 Inadunata, Camerata, and Articulata. The plates are then almost 



