vin ECHINODERM AT A MORPHOLOGY OF SKELETON 325 



The posterior group consists of the two posterior radial plates ; the posterior 

 unpaired (fifth basal) plate is wanting. The anterior group is separated from the 

 posterior by a row of plates which belong to the right and left posterior interradii, as 

 can be seen by comparing the figures. This arrangement is found in no other Echi- 

 noid. As in all Echinoids, however, the radials remain connected with the apical 

 ends of the five double rows of ambulacral plates, so that these latter divide in a 



V 



FIG. 285. Apical system and neighbour- 

 ing part of the perisome of Pourtalesia 

 Jeffreysi, W. Th. (after Loven). For letter- 

 ing see p. 317. 



FIG. 284. Apical system 

 of Collyrites elliptica, Lam. 

 (after Loven). For lettering 

 see p. 317. 



remarkable manner into three anterior ambulacra (trivium), the anterior unpaired 

 and the anterior right and left, and two posterior ambulacra (bivium). 



4. The apical system is most of all reduced and obliterated in the peculiar 

 Spatangoid family of the Pourtalesiidce (Fig. 285). Let us take as an example P. 

 Jeffreysi. The whole system, which is irregularly pentagonal in outline, is shifted 

 forward, and separated from the apical ends of the two posterior ambulacra by 

 the uppermost plates of the posterior unpaired and of the right and left posterior 

 interradii. It, almost certainly, consists of four basal plates, each perforated by a 

 genital pore, but fused together into one single piece in which no suture can be seen. 

 In the central and anterior portion of this plate lie the scattered pores of the stone 

 canal. No radials can be recognised. 



