364 CLYPEUS 



The poriferous zones are broadly petaloidal on the dorsal surface ; at the border the 

 pores are as closely approximated as they are at the base (PL XXIX, fig. la,6); about 

 two lines up the sides they become abruptly apart, and continue so throughout the zones ; 

 they again gradually approximate in the vicinity of the disc (fig. I a, d) ; the holes of the 

 inner row are round, those of the outer row are slit-like, and the outer and inner holes of 

 each pair are united by fine fissures (PL XXIX, fig. 1 c) ; the intermediate septa having a 

 single row of small granules on their surface ; at the base the pores forming a pair are 

 small, and closely approximated ; the zones are very narrow, and form wavy lines, in 

 which the bigeminal pores are placed at intervals apart, sometimes they are disposed 

 in rectilineal order, and sometimes arranged in triple oblique pairs ; at PL XXIX, 

 fig. 1 ^, I have represented this arrangement of the pores in one of the basal 

 zones, where each small ambulacral plate is seen to be perforated by a pair of pores. As 

 the ambulacral plates are much narrower on the upper surface than they are at the 

 base, the pores are more numerous, and more closely approximated on the dorsal surface, 

 where there are from eight to ten pores opposite each large plate (PL XXIX, fig. 1 c). 



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

 the single area is the widest, and the posterior pair of intermediate dimensions; there 

 is often a slight depression down the centre of each area in the line of the median suture, 

 which, with the flat petaloidal depression formed by the poriferous zones, produces a series 

 of undulations on the upper surface of the areas (PL XXIX, fig. 1 6). The single inter- 

 ambulacrum is deeply cleft by the anal valley, which commences narrow at the disc, 

 expands towards the middle, and still more so at the border (PL XXIX, fig. I a). The 

 sides are vertical, and the vent is seen at the extremity of the channel, and is figured in 

 shadow in fig. 1 a. The border of the test forms a series of graceful undulations, 

 from the prominence and convexity of the inter-ambulacral and the narrowness of 

 the ambulacral areas. (PL XXIX, fig. 1 a and fig. 1 ^, shows the undulation of the 

 border.) 



The tubercles on the upper surface are small, and nearly equal in size throughout 

 both areas ; those in the ambulacra are arranged in oblique V-shaped rows, and those on 

 the inter-ambulacral plates in nearly horizontal rows, of which there are five or six on 

 each large plate (PL XXIX, fig. 1 c). At the base, they are more unequal in size, and 

 larger than on the upper surface ; in the middle of the area they are smaller ; at the sides 

 they are larger, and sometimes the large tubercles are perforated, as shown at fig. 1 e and 

 fig. If. They are surrounded by deep areolas, and between these depressions the 

 surface of the plates is covered with fine granulations (fig. 1 e). It is right to remark, 

 that in the specimens I have hitherto examined the perforation of the large basal tubercles 

 is an exceptional, rather than a general character. 



The apical disc is large, and lies in a depression behind the vertex. Its plate-elements 

 are rarely seen separate, except in specimens which have been decomposed, as in 

 this urchin, the genital and ocular plates are all firmly anchylosed together from early age ; 



