4 ASTHEXOSOMA HYSTRIX. 



line la but little larger than the number of interambulacral plates. On the 

 actinal side fifteen of the former occupy the same space as twelve of the 

 latter, and on the abactinal aide the proportion is thirteen to ten. On PL 

 II /'. :,' of the Revision of the Echini 1 have given an internal view of a 

 portion of the test, showing the lapping of the plates of the two areas in 

 opposite directions, and the arrangement of the poriferous zone; the median 

 ambulacra! part of the plates lap from the actinostome ; the median interam- 

 bulacra and the plates of the buccal membrane towards the mouth. Some- 

 thing similar is found already in Astropyga. where we have a greater degree 

 of flexibility in the coronal plates of the actinal surface than in other Diade- 

 matidae. 



There is on the actinal side, adjoining the poriferous zone, a row of large 

 primary interambulacral tubercles, occupying the outer extremity of the 

 plate; these large tubercles are surrounded by a Hat scrobicular circle, raised 

 above the edge of the te.-t, with sunken BCrobicular area. This row extends 

 to the ambitus, ami becomes very irregular on the abactinal side of the test ; 



a few smaller tubercles, larger than those of the rest of the plates, extend 

 towards the apical system. On each side of the median interambulacral 

 line, on the actinal side, a smaller vertical row of tubercles, less distinct, and 



at a greater distance from the edge of the plate, extends somewhat beyond 

 the ambitus. A similar row of tubercles extends in the median ambulacra! 

 space, on each side of the median line, reaching hut little beyond the am- 

 bitus. The remaining part of the coronal plates is occupied, both in the 

 ambulacra] and interambulacral space-. ly small secondary tubercles and 

 miliaries, irregularly arranged, with deeply sunken scrobicular areas and 

 raised edges, as in the primary tubercles, forming, in the case of the smallest 

 miliary tubercles, deep pits.* On the abactinal side the pits form irregular 



horizontal lines, parallel to the BUtures, in the central part of the plates. 

 The whole abactinal system is similarly pitted by secondaries and miliaries. 



Tin- anal system is large, composed of rather irregularly shaped polygonal 

 plates; the genital plates are triangular, small, with genital openings near 



the outer edge. The ocular plate- are -mall, polygonal, reaching the anal 



Bystem j they are separated from the genital plate- \>\ anal plates, one on 



i ■■■ examine (from the interior) the actinal 

 the primary tubercle*, which 

 Bra hollow; thii name itrnrtnre, more foil} developed, forma the mnken tcrobicular areas of Asthi 

 among Spatangoidi ii i- highrj developed in Lovcuia, where tin' ranken areai form pouches in portioni 

 nf the interambnlai r.'i 



