18 NACOSPATANGUS GRACILIS. 



consists of unifoiTQ tubercles, slightly larger towards the ambitus, and 

 towards the edge of the actinal part of the ambulacra. Distinct subanal 

 fasciole, with rudimentary anal branches. General facies that of Micraster, 

 also of the high test species of Maretia and Spatangus. Ambulacra flush with 

 the test ; poriferous zones of the petals slightly sunken. Spines of the abactinal 

 surface curved at the base, short, rather stout for a Spatangoid, of uniform 

 length. ' On the abactinal surface they increase in length according to the 

 prominence of the tuberculation. 



Nacospatangus gracilis 



! Nacospatangus gracUis A. Ac, 1873, Bull. M. C. Z., III. p. 189. 



PL II. f. 3-5. 



Test thin, seen from above elliptical, rounded anteriorly, slightly angular 

 laterally, tapering towards the posterior extremity, which is truncated. In 

 profile the anterior extremity arches regulai^ly towards the vertex, which is 

 posterior, half-way between the apical .system and the nearly vertical anal 

 extremity. The anal system is transversely elliptical, covered by two outer 

 concentric rows of larger plates, each carrying one or two minute secondary 

 tubercles, with smaller ones round the central anal opening. Nearly on the 

 same vertical plane with the anal system, sloping slightly towards the actino- 

 stome, is situated the large subanal plastron. This terminates in a beak at 

 the junction with the keel of the actinal plastron, and is edged with a distinct 

 narrow fasciole, sending an indistinct rudimentary branch from the rounded 

 anal extremity of the plastron, parallel with the anal system. The actinal 

 plastron is narrow, elongate, keeled, flanked by broad bare ambulacral 

 avenues. The mouth is large, transverse, with a prominent posterior lip. 

 The tubercles of the actinal surface are largest on the edges of the ambula- 

 cral avenues, diminishing in size towards the ambitus, where they gradually 

 l)ass into the nearly uniform tubercles of the abactinal surface. The spines 

 are silver-gray, rather stout, curved at the base ; they cover thickly the 

 whole abactinal surfice ; they are longer on the actinal side, according to 

 the size of the tubercles, and largest on the edge of the ambulacral avenues. 

 Miliaries and secondaries distant, in-egularly scattered between the primaries; 

 on the abactinal surface less numerous towards the ambitus. The apical 

 system is anterior ; three large genital openings placed close together ; right 

 anterior one wanting. The median interporiferous zone of the odd ambula- 



