AR^OSOMA BELLI. 189 



justify the recognition of this small West Indian species. The largest 

 specimen before us is only 105 mm. in diameter. 



This species is readily distinguished from A. hystrix by the greater 

 height of both the ambulacral and interambulacral plates. In a specimen 

 of A. Belli 72 mm. in diameter there are 22 interambulacral and 27 am- 

 bulacral plates in each column, from the ambitus to the abactinal system. 

 On the actinal side, 16 interambulacral and 17 ambulacral plates in a 

 column lie between the ambitus and the actinal system. 



In a specimen of the same species 105 mm. in diameter there are 27 

 interambulacral and 40 ambulacral plates between the ambitus to the 

 abactinal system. On this actinal side there are, from the ambitus to 

 the actinal system, 18 interambulacral and 18 ambulacral plates. 



In a specimen of A. hystrix 130 mm. in diameter we find on the abactinal 

 side 27 interambulacral and 38 ambulacral plates between the abactinal 

 system and the ambitus, and on the actinal side between the actinal system 

 and the ambitus there are 22 interambulacral and 33 ambulacral plates. 



In A. Belli there are two principal columns of interambulacral pri- 

 maries on the actinal side, one of which includes a primary on each plate 

 adjoining the ambulacral system ; the other is nearer the median line, and 

 includes only one on every other plate. These columns extend but little 

 beyond the ambitus ("Hassler" Ech., PI. II, figs. 1, 2); a secondary column 

 of distant and irregularly placed primaries extends from the ambitus to the 

 abactinal system. The ambulacral and interambulacral plates each carry 

 one irregular row of small secondaries and miliaries at the centre of the 

 plate. 



In A. hystrix there are on the interambulacral plates six or seven 

 vertical rows of primaries and secondaries, four of which are more prominent 

 and regular than the others. In the median ambulacral area there are 

 two series of primary tubercles. The rest of the plates, in both areas, are 

 thickly covered with miliaries and a few very small secondaries. 



The arrangement of the primaries and secondaries is much the same on 

 the abactinal side, with the exception that the primary tubercles are 

 smaller. 



There are, both in A. Belli and A. hystrix, on the actinal side, spines 

 with hoofs and many with a flaring extremity. 



