PHORMOSOMA HISPIDUM. 



87 



pairs of ambulacral buccal plates, and three to four ambulacral plates to 

 each interambulacral plate at the ambitus, and four and five above (Pis. 48, 

 fig. 2 ; 49, fig. l), and two to four in the actinal plates above the primor- 

 dial plate. There are three and four interambulacral plates from the pri- 

 mordial plate to the ambitus and five and six to the abactinal system. The 

 interambulacral plates each carry a primary tubercle (PL 49, fig. l). The 

 primordial plate has a second small one ; these are, below the ambitus, 

 placed near the ambulacral edge, above the ambitus, in the central part 

 of the plate. In addition, on the actinal side, each 

 plate carries from one to three miliaries on the inner 

 side of the primary. On the abactinal side the three 

 plates nearest the ambitus carry, parallel to the sutures, 

 one or two small primaries in addition to the two or 

 three miliaries. The two last ambulacral plates carry 

 only a single miliary. 



In the odd anterior ambulacral area (PL 48, fig. ~) 

 the first four and three abactinal ambulacral plates, 

 Fig. 133, are arranged in a linear series; the fifth and 

 fourth are much larger, and are followed by two 

 smaller plates reaching to the median line. The next 

 large plate is followed by one reaching the median 

 line and one which is excluded ; and then, as far as 

 the ambitus, of the succeeding three plates only the 

 larger reaches the ambitus; the others are excluded, 

 of the three plates which go to form the ambulacral system, the third 

 plate only is excluded, the second plate reaching the median line (PI. 48, 

 fig. J). Nearly all the larger ambulacral plates which extend from the 

 interambulacral edge to the median line carry a primary tubercle or a 

 large miliary, and six of the excluded secondary plates carry similarly a 

 small miliary. Of the younger series of abactinal ambulacral plates only 

 the fourth has a miliary. Of the buccal plates the two oldest plates carry 

 one or two miliaries. 



The abactinal system is 6 mm. in diameter (PL 40, fig. 1). At this 

 stage the ring of genital and ocular plates is closed, excluding the anal 

 system from the interambulacral plates. The madreporic genital is some- 

 what larger than the others, and carries but two miliaries, it is more rounded 

 than the others, which are irregularly pentagonal, and carry in the central 



15 linn. 



Fig. 133. Ph. Hispiimm. 



Below the ambitus. 



