32 



Mr. W. K. Spencer. O'n the Structure and [Mar. 22, 



the interradial perisomatic plates to those of Echinocystis, and the- 

 characters of the masticatory apparatus, are all points which ally 

 Palseodiscus to the Echinoidea, and separate it from the Stelleroidea."' 

 On two other points, however, his description is by no means so 

 accurate, for he says that the anibulacral plates are flush with those of 

 the interradii, and the anibulacral plates are not perforated by pores, 

 but the podia pass out between the plates. 



The most perfect specimens of Palceodiscus ferox discovered up to the- 

 present are in the Grindrod collections of the Oxford University 

 Museum. Using these as his material, Professor Sollas, in 1900, gave 

 a description of the ambulacra and the lantern of Aristotle. All the 

 specimens except one were in the form of casts. By the aid of these, 

 casts and a well-preserved ambulacrum two series of plates in the 

 ambulacra were identified, an outer series, pierced for podia, and r,n 

 inner series. 



Professor Sollas (20, p. 705) called the outer series the true Echinoid 



1 KXT FIG. 1. Ventral surface of Palceodiscus ferox reconstructed from several 

 specimens in the Oxford Museum ; i.iap., initial interambulacral plate ; 

 am.p., ambulacral plates; rh.p., rhomboidal plates of interambulacral 



