viii ECHINODERM ATA— MORPHOLOGY OF SKELETON 



^m 



(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 



cjLv 





■ ICl/tV 



Fig. 2S9.— Apical system of Cyatho- 



crinus. For lettering see p. :!17. ("/(, Anal 

 interradial. 



Fio. ■200.— Marsupites ornatus. Plates of 

 the ilorsal eup. 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 ; (h) among the Camerata, the 

 families of the lieteocrinida' p. p., 

 Iihodocrinidce, Gli/ptasterida', and 

 Crotalocrinidce ; (c) the Articidata 

 (Ichthi/ocrinidce) ; (d) the Canalicu- 

 laia, 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 

 Pseudomonocyelie. 



The Crinoids with monoeyclie 

 base (without infrabasals, Fig. 291) 

 are, apart from a few Inadunata, 

 the Camerate families of the 

 Melocrinida', Adinocrinida', Platy- 

 crinida', Hexacrinidce, Acrocrinida^, 

 Barrandeocrinidce,, EucMlyptocrinida. 



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 Artirulata. The plates are then almost 



Fig. 291.— Actinocrinus proboscidalis. Plate 

 of the ilorsal cu]!. For lettering see p. 317. 



