VIII 



ECHINOVERM ATA— MORPHOLOGY OF SKELETON 35!) 



(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 



<is 



^ ^ 



#(^i 



Fig. iS9.— Apical system of Cyatho- Fin. 2fio.— Marsupltes ornatus. Plates of 



crlnus. For lettering .see p. 317. ta«, An.%1 the dorsal cup. For lettering see p. 317. 



interradial. 



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

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



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

 and 290) are: {a) most Inadunata , (h) among the Camerata, the 

 families of the lieteocrinichi' p. p., 

 Ehodocrinidce, Gb/ptasterida', and 

 Cwtcdocrinidm , (c) the Articidata 

 (IcMhyocrinidce) ; (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 convenientlj' termed 

 Pseudomonoeyelie. 



The Crinoids with monoeyelie 

 base (without infrabasals, Fig. 291) 

 are, apart from a few Inadunata, 

 the Camerate families of the 

 Melocrinida, Adinocrinidm, Platy- 

 crinidce, Hexacrinidce, Acrocrinida, 

 Ba.rrandeocrinidce, Eucidyptocrinidce. 



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 Artkulata. The plates are then almost 



Pig. 291.— Aotinoorinus proboscidalls. Plate 

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



