VIII ECHINODERMATA— MORPHOLOGY OF SKELETON 



369 



radials, interdistiehals. etc. are found binding the brachials into the rigid horizontal 

 border. Their number and arrangement are best elucidated by the figuie. The anal 

 interradius is not distinguished from the others in any marked manner. 



b. The Tegmen calycis. — The tegmen of Marsupiocrinus [cmlatus] is only sligiitly 

 vaulted. It is plated with numerous small, firmly connected pieces (Fig. 321). 

 Among these, we can easily distinguish the covering plates of the ambulacra, which 

 thus here come to the surface, and 

 • can easily be distinguished from 

 the somewhat larger interradial 

 and interambulacral plates. In 

 the centre of the tegmen lie the 

 five orals, arranged in the manner 

 which is characteristic of the 

 Caiiieratd,, and behind these, 

 subcentrally, the anal aperture, 

 surrounded by special plates. 



If the ainbulacral covering 

 plates are larger and more massive, 

 as in many species of the genus 

 Platycrinus, it is then more 

 difficult to distinguish the inter- 

 radial plates of the tegmen from 

 them. 



The genus Agaricocrinus 

 affords examples of the specially 



strong develojmient of single covering plates of the ambulacral skeleton, which are 

 called radial dome plates. The tegmen is highly vaulted. 



An e.xtraordinarily highly vaulted tegmen is found in the Adinocrinidcc 

 {Actinoerinus, Batocrinus, Figs. 253, 254, pp. 307, 308). It is regularly and firmly 



Fio. 321.— Tegmen calycis of Marsupiocrinus coelatus 

 (after Wachsmuth and Springer), or, Orals; um, am- 

 bulacra ; cp, covering;- platPs of the ambulacral furrows ; 

 ia, interambulacral region. 



CSi 



V Wo 





Fig. 322.— Part of the dorsal cup of Forbesiocrinus, spread out. For lettering see p. 317. 

 In addition, lO, one of the four similar interradial regions; lA, tlie deviating anal interradial 

 region ; pal, palmars. 



plated with large strong plates more or less equal in size. Nothing can be seen 

 of the ambulacral skeleton externally, it having been pressed down, or rather, over- 

 grown, as already described, by the interambulacral plates. In the posterior inter- 

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