viii ECHINODERMATA 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. 289. Apical system of Cyatho- 

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

 interiadial. 



Fin. 29U.--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 dicyelie base (with infrabasals, Figs. 289 

 and 290) are: (a) most Iitaduimta ; (l>) among the Camerata, the 

 families of the Rdeo<-riui<l<i' p. p., 

 Bhodocrinidce, Glyptasteridce, and 

 Crotalocrinidce ; (c) the Articulata 

 (Ichthyocrinidce) ; (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 

 Pseudomonocyclic. 



The Crinoids with monoeyclie 

 base (without infrabasals, Fig. 291) 

 are, apart from a few Inadunata, 

 the Camerate families of the 

 Melocrinidce, Actinocrinida>, Platy- 

 crinidce, Hexacrinidce, Acromnidce,, 

 JBarmndeocrinidce, Eucalyptocrinida.'. 



Instead of the typical five infrabasals and five basal s 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 Arficuhita. The plates are then almost 



FIG. 201. Actinocrinus proboscidalis. Plates 

 of the dorsal cup. For lettering see p. 317. 



