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adambulacral plates arc mostly two in number, occasionally three 

 and very rarely four. In the last case the spines are of subequal 

 size and much smaller than when there are less. There are in 

 addition some exceedingly small spines on the actinal surface of 

 the adambulacral plates. 



Mouth-plates. — The mouth-plates (PI. I, fig. 3) are compara- 

 tively large and are similar in general form to the adambulacral 

 plates, the two plates of a pair together forming a trapezoid with 

 the base turned towards the mouth and separated from each 

 other by a space covered over with a membrane. Each plate has 

 the form of a nearly right-angled triangle wdth the right angle 

 apposed to that of its fellow plate. On the side turned towards 

 the mouth there is a seires of 4-6 spines, gradually decreasing in 

 size towards the first adambulacral plate ; on the actinal surface 

 of the plate there is a more or less regular series of some five or 

 six spines similar to the large spines of the adambulacral plates 

 and gradually becoming smaller away from the mouth. 



Ventrolaterals. — The ventrolateral plates (PI, I, fig, 5, 6) are 

 confined to the disk, and there are only about a dozen plates in 

 each interradius, forming two arc- shaped series between the in- 

 feromarginals and the adambulacral plates. The inner series i.e. 

 the one nearer the centre of the disk, consists of some nine plates, 

 of which the two on either side of the interradial lino are larger 

 than the others and bear each one or two long spines similar to 

 those of the inferomarginals but smaller. The next pair of plates 

 are also tolerably large and bear each a similar spine ; the 

 remaining plates of the inner series are small and bear a number 

 of exceedingly small spines without any regular arrangement ; 

 these small spines being also found sometimes on the larger 



