302 HYBOCLYPUS. 



an anteal elevation of the test, which attains its greatest development in H. gibberulus. 

 (See fig. 2/, k.) 



The ambulacral areas have all nearly a uniform width ; the single and anterior pair 

 are nearly straight, and converge around the front part of the disc (fig. 1 a) ; the posterior 

 pair are a little bent, and curve upwards, inwards, and forwards, to terminate at 

 the posterior lateral part of the disc, their apices just falling within the longitudinal valley. 



The apical disc is large, and formed of seven ovarial and five ocular plates ; the 

 two pairs of ovarials are perforated ; the single plate is imperforate, and composed of three 

 pieces (fig. 1 e) ; the pairs of ovarials expand between the apices of the ambulacra, and the 

 posterior elements of the single plate bend over the upper part of the longitudinal valley 

 (fig. 1 a) ; the madreporiform body is round, button-shaped, and placed between the 

 anterior and the right antero-lateral ambulacra ; it therefore rests on the surface of the 

 right ovarial plate, as in other Echinid^e. The single ocular plate is small ; the anterior 

 pair are large, and placed side by side in the centre of the disc, between the anterior and 

 posterior pairs of ovarials ; but the posterior oculars are not seen in situ in any of my 

 specimens. 



The base is concave, and much undulated, from the convexity of the basal portions of 

 the inter-ambulacral areas ; the mouth opening is small and excentral, it has an oval form, 

 its long diameter being in the direction of the antero-posterior diameter of the body ; 

 around the border of the opening the poriferous zones become closely crowded with pores, 

 which lie in triple oblique pairs, and form a radiate rosette around the orifice. 

 These areas are well seen in fig. 1 b, and a portion of one of them, magnified, is drawn in 

 fig. If, where each of the broad plates are seen to be perforated with a pair of pores, 

 having one, two, or three large tubercles on their surface. 



There are four or five rows of small tubercles, so arranged on the plates that they 

 form, as in most of the Echinoconid^e, oblique V-shaped lines (fig. 1 d). The poriferous 

 zones are narrow, and on a level with the general surface of the test ; the pores are placed 

 close together on the dorsal surface, there being six pairs of pores opposite one inter- 

 ambulacral plate ; at the base they are wide apart, from the increased breadth of the 

 ambulacral plates in this region (fig. 1 b and/). 



The inter-ambulacral areas are five or six times the width of the ambulacral 

 (fig. 1 a and b) ; they are, however, of unequal width ; the anterior pair are the narrowest, 

 the posterior are wider, and the single area is the widest; in its upper part is the 

 longitudinal valley, which is very deep above, with vertical walls, that expand in the lower 

 half, and form a concave depression (fig. 1 a) ; the posterior border, which is very little 

 produced, is rounded, or only slightly flattened (fig. 1 a and b), and not truncated, as in 

 H. caudatus and H. gibberulus. The tubercles are small, and very numerous (fig. 1 d) ; 

 they are arranged in concentric rows, and I have counted fifty tubercles on the 

 third plate, above the border ; they are surrounded by sunken areolas, and the inter- 



