76 I \l.\M0CKl\rs IMOMKD.K. 



What is (.'specially interesting to note here is. that the firs! formed anal 

 plates OCClipy a radial position ; that is. they occupy the same position 

 which the 80 called infrabasals do in Ophiurans and Starfishes according to 

 Carpenter and Sladen. They are also within the ring of genital plates. 

 This radial position the live original plates retain apparently, and they can 

 still he traced in all the figures of Cidarida- here given, in spite of the in- 

 tercalated plates separating them, or in spile of the formation of a second, 

 third, or fourth inner ring of anal plates. It is interesting to note that 

 in the oldest Sea-urchins, both in Bothriocidaris and in Palaechinus, the 

 position of five of the plates of the outer anal ring is radial, and we may 

 fairly assume that they have been separated by a set of five other plates 

 and an outer anal ring of ten plates formed very much after the manner 

 described for the young CidaridaB of to-day. 



Owing to the irregularity in the development of the inner anal plates, 

 the first outer ring does not always consist of the same number of plates. 

 It varies greatly according to the time which the inner plates have required 

 to reach the periphery. 



In Dorocidaris papillata (Plate XXIX. Figs. 9, 10) we have an outer 

 ring of anal plates consisting of ten plates. In Phyllacanthus baculosa 

 (Fig. 7) we have eleven anal plates forming the outer ring, only one plate 

 of the second ring having as yet found its way to the outer one; while 

 in Dorocidaris Bartletti (Fig. 12) we find two plates separating the adjoining 

 five radial anal plates. In Porocidaris Sharreri (Fig. 5) only two of the five 

 radial anal plates are separated by two plates ; and in Dorocidaris Blakei 

 (Fig. 11) four of the radial anal plates are separated by two plates. 



This has an important bearing on the number of the anal plates we 

 find in the Aspidodiadematidae and in the Echinothurige, in both of which 

 families the genital and ocular plates form a single ring around the anal 

 system, which varies from five to eight. See Figures 13-15 of Plate XXIX. 

 In the case of Aspidodiadema tonsum (Fig. 13) it will be noticed that the 

 five anal plates are placed radially. In the case of Aspidodiadema micro- 

 tuberculatum there are two rings of anal plates, each of ten plates (Fig. 16). 



We must also remember that the genital and the ocular plates of the 

 oldest known Echini formed a single ring, practically on one level, and 

 that it is only among more recent Echini that we find radially a consid- 

 erable distance between the base of the anal edge of the genital ring and 

 the tip of the intercalated oculars. In the recent representatives of the 



