vin ECHINODERMATA MORPHOLOGY OF SKELETON 369 



am, 



CjO 



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

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

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



b. The Tegmen calycis. The tegmen of Marsupiocrinus (coelatus) is only slightly 

 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 

 Camerata, and behind these, 

 subcentrally, the anal aperture, 

 surrounded by special plates. 



If the ambulacral 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 Fu) ' 321. Tegmen calycis of Marsupiocrinus coelatus 

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



bulacra ; cp, covering plates of the ambulacral furrows ; 



The genus Agaricocrinus i a , interambulacral region, 

 affords examples of the specially 



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

 called radial dome plates. The tegmen is highly vaulted. 



An extraordinarily highly vaulted tegmen is found in the Actinocrinidce 

 (Actinocrinus, Batocrinus, Figs. 253, 254, pp. 307, 308). It is regularly and firmly 



an. 





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

 In addition, IO, one of the four similar interradial regions ; iA, the 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- 

 VOL. II 2 B 



