4 ASTHENOSOMA HYSTRIX. 



line is but little larger than the number of interambulacral jilates. On the 

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

 latter, and on the abactinal side the pi-oportion is thirteen to ten. On PI. 

 IP f. 5" 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 

 ambulacral 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 Astrojiyga, 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 

 jilate ; these large tubercles are surrounded by a flat scrobicular circle, raised 

 above the edge of the test, with sunken scrobicular area. This row extends 

 to the ambitus, and becomes very ii-regular on the abactinal side of the test; 

 a few smaller tubercles, lai'ger than those of the rest of the plates, extend 

 towai'ds 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 ambulacral 

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

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

 ambulacral and interambulacral spaces, by 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 sutures, in the central part of the plates. 

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

 The anal system is large, composed of rather irregularly shaped pol3'gonal 

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

 the outer edge. The ocular plates are small, ])olygonal, reaching the anal 

 system ; they are separated from the genital plates l)y anal plates, one on 



* Something analogous to this is found in Astropyga. If we examine (from the interior) the actinal 

 surface of a large Astropyga, we find deep pits extending into the Ijase of the primary tubercles, which 

 are hollow ; this same structure, more fully developed, forms the sunken scrobicular areas of Asthenosoma ; 

 among Spatangoids it is highly developed in Loveuia, where the sunken areas form pouches in portions 

 of the interambulacral areas. 



