706 ME. T. T. GROOM ON" SOME 



boundary of the compound plates is occupied by a primary ; then 

 come two demiplates, the pores of which form with those of the pri- 

 mary one of the three oblique rows. The largest primary succeeded 



Fig. 2. — Two Ambulacra! Plates from the Equator of 

 Pelanechinus corallinus. x 4. 



i 



\ 

 i 

 i 



a, aboral ; b, adoral boundary. 



Fig. 3. — Ambulacral Plates from above the Equator of 

 Pelanechinus corallinus. x4. 



by two demiplates forms the second oblique row, while the aboral 

 primary with two demiplates forms the third. At one place (fig. 2) 

 where some of the pores are aborted, the boundaries of the demi- 



