376 CLYPEUS 



prominent undulations between the narrow ambulacra (fig. 1 b, c). The ambulacral 

 areas are narrowly lanceolate^ the anterior pair curve gently upwards and outwards, and the 

 posterior pair upwards and inwards ; the poriferous zones are narrowly petaloid three 

 fourths of their length between the border and disc ; at the lower fourth the pores forming 

 a pair are closely approximated, and the rows are very narrow ; above this point the pores 

 of the inner row are round, those of the outer row in the form of oblong slits 

 (fig. 1 e), which are connected with the inner row by fine sutures ; the transverse sulci dis- 

 appear some distance above the border, the pores then become simple, oblique, and wider 

 apart ; in the basal portion of the zones the pores are minute and far apart, and their 

 track is only traced by the depression formed by the ambulacra near the mouth-opening ; 

 around the peristome (fig. 1 b) the pores become more numerous, and form five leaf-like 

 expansions. The inter-ambulacral areas are of unequal width, the anterior pair are the 

 narrowest, the single area the widest, and the posterior pair of unequal dimensions (fig. 1 a); 

 the anterior border is obtusely rounded, the sides swell gradually outwards to the junction 

 of the posterior pair with the single area, the widest part of the test is in the direction of a 

 line passing transversely across the vertex ; the single inter-ambulacrum is slightly produced, 

 deflected, and truncated (fig. 1 a, d) ; the anal valley is short and wide, and occupies the 

 lower half of the inter-ambulacrum (fig. 1 a, c, d) ; between the upper portion of the 

 valley-arch and disc the test is undepressed (fig. 1 a) ; the vent occupies the extreme 

 termination of the valley, and its perpendicular sides are scooped out (fig. 1 d) to afibrd 

 greater space for the passage of the intestinal tube and the closure of the aperture by its 

 circle of anal plates (PI. XLI, fig. 1). 



The apical disc is small, and its elements so intimately soldered together that their 

 separate study is impossible in the specimen figured ; the disc, moreover, is so much 

 covered over by the madreporiform body that the sutures are all concealed (fig. If). In 

 another small specimen the disc is composed of two anterior and two posterior perforated 

 ovarial plates, and a single imperforate ovarial ; the five ocular plates are very small 

 and their orbits marginal. 



The test is moderately thick, and the surface of the plates is covered with several 

 horizontal rows of small tubercles (fig, 1 e) ; the base of each is encircled by a sunken 

 areola, and the intermediate portion delicately sculptured with microscopic granules 

 (fig. 1 e) ; the tubercles at the base are only a little larger than those on the upper surface. 



The base is flat, slightly concave, or undulated, the ambulacra form straight 

 valleys, and the inter-ambulacra moderately convex elevations ; the prominence of these 

 undulations and the deflection of the inter-ambulacrum are greater in proportion in small 

 individuals. The mouth-opening is excentral, being situated near the junction of the 

 anterior with the middle third of the antero-posterior basal diameter ; the pentagonal 

 peristome is surrounded by five lobes, and the terminations of the ambulacra form five 

 leaf-like expansions, or a penta-phylloid floscule in which the pores are arranged in triple 

 oblique ranks. 



