320 COLLYRITES 



and are nearly half as deep as the inter-ambulacral plates (fig. % b). In the posterior pair 

 the plates on the sides are much deeper, for in these areas there are not more than two 

 or three plates opposite one large inter-ambulacral plate. The left postero-lateral 

 ambulacral area extends farther forward than the right. 



The inter-ambulacral regions are large and of unequal width ; the antero-lateral areas 

 are the narrowest, the single area is of the same width, and the postero-lateral are one 

 third wider than the anterior pair ; the large pentagonal plates composing these areas are 

 bent in the middle, and there are from fifteen to sixteen pairs in each ; the plates carry 

 three or four concentric rows of small tubercles (fig. 2 e), numbering from fourteen to 

 sixteen on each plate ; the tubercles which are perforated are surrounded by well-defined 

 areolasj raised on crenulated bosses (fig. 2 g), and the inter-tubercular surface is crowded 

 with microscopic mihary granules (fig. 2 g) ; the basal are larger than the dorsal tubercles, 

 especially those situated on the prominent portion of the single inter-ambulacrum ; some 

 of these have wide hexagonal areolas, closely set together (fig. 2 h). 



The poriferous zones are narrow ; in the anterior zones on the sides there are about six 

 pairs of pores opposite each large plate (fig. 2 e), whereas in the posterior zones in the 

 same portion of the test there are only three or four (fig. 2 (f). At the base the pores are 

 small, very indistinct, and placed widely apart. 



The base is flat at the sides and before, but is convex in the region of the single inter- 

 ambulacrum. As the basal portions of the ambulacral areas are nearly destitute of tubercles, 

 and the basal portions of the inter-ambulacral areas are furnished with larger tubercles, 

 the course of the former is readily made out by the comparatively naked track formed by 

 them, from the border to the mouth (fig. 2 b). 



The small circular mouth-opening is situated in a depression at the anterior fourth of 

 the base, the peristome is entire, and there is no trace of notches nor of any armature 

 for the mouth (fig. 2 b). 



The apical disc is placed rather nearer the anterior than the posterior border (fig. 1 a). 

 It has an elongated figure, but its elements are so intimately soldered together that their 

 relative anatomy is with difficulty made out. In most specimens the anterior ovarials 

 project between the single and the antero-lateral ambulacra (fig. 2/) and the right plate, 

 which is the largest, supports the madreporiform body on the same line. Behind the 

 anterior ovarials the anterior pair of ocular plates are placed, and behind them come the 

 posterior pair of ovarials, with the single imperforate ovarial behind and between them ; 

 the small single ocular plate is seen at the apex of the single ambulacrum, but the position 

 of the posterior oculars I am unable to determine, notwithstanding the perfect con- 

 servation of one of my specimens. 



The posterior border is slightly truncated; at its upper part, and nearly midway 

 between the basal angle and the apex of the posterior ambulacra, the anal aperture 

 is situated (fig. 2 d). It is a small oval opening, the longest diameter of which is 

 in the direction of the longitudinal axis of the test. The basal portion of the single 



