Pig. 9. — Transverse section tlirougJi an amhulacrum of 

 Palseodiscus ferox ( X 12).- 



[The uppermost line was obtained from a transverse section of a wax im- 

 pression of a hollow cast : the remaining lines are restored from a general 

 study of the ambulacra. The thin lines are entirely hypothetical.] 



Eigs. 10 & 11. — Amhulacrum of Palaeodiscus ferox : hollow casts 

 from the vicinity of the mouth and the ambitus respectively ( X 12). 



r- 



[The lowermost figure shows the median zigzag sutural line of the outer or 

 Echinoid ambulacra! plates, on a central ridge formed by the sandstone which 

 fills the hollow of the ambulacral groove. The more darkly shaded areas repre- 

 sent the cavities left by the inner or Asteroid series of ambulacral plates : 

 the light circular processes at the margins of these represent one pore of a pore- 

 pair — its fellow is not very conspicuous in these casts.] 



