FROM THE INFERIOR OOLITE. 279 



sides, which are pentahedral, rise abruptly from the marginal fold; the anterior and 

 lateral pairs of inter-ambulacra form angles of from 50° to 55° with the base, and the 

 single inter-ambulacrum makes an angle of about 42° (fig. 2 a). The ambulacral areas 

 are narrow and prominent, and placed nearly equidistant from each other ; the single 

 anterior area is straight ; the anterior pair curve forwards, upwards, and backwards ; the 

 posterior pair rise forwards and upwards for three parts of their course, then curve 

 inwards towards the anal opening, and terminate near the posterior part of the disc 

 (fig. 2 a). There are two complete rows of tubercles extending from the peristome to 

 the disc, arranged on the margins of the areas, a primary tubercle being developed on every 

 third plate (fig. 2 d) ; two inner rows commence near the mouth, pass round the angle, 

 and rise half way up the sides, but the tubercles thereof are smaller, less regular in their 

 arrangement, and more incomplete than the marginal rows ; like them, however, one 

 tubercle rises from every third plate ; at the widest part of the areas, near the border, 

 a few tubercles form a fifth row of very limited length (fig. 2 b). 



The poriferous zones are narrow, and sunk in slight depressions (fig. 2 c) ; the pores 

 are strictly unigeminal throughout ; the pores are nearly equal, and the septa develop 

 a small ridge between them (fig. 2 d) ; one pair of pores corresponds to a pair of ambulacral 

 plates ; as there are thirty marginal tubercles on the upper surface of one area, and every 

 third plate supports one tubercle, it follows that there are ninety pairs of pores in the 

 poriferous zones on the upper surface alone ; as the areas are only partially exposed at the 

 base, the pores cannot be counted in that region. 



The inter-ambulacral areas are more than four times as wide as the ambulacral ; there 

 are eighteen pairs of plates in each area, between the margin and the disc ; each plate is 

 bent in the middle (fig. 2 d), and supports numerous small tubercles, their number on the 

 plates varying according to their length ; they are disposed in single transverse rows on 

 the centro-sutural half, and in double rows on the zonal half of the plates ; near the 

 margin, there are four tubercles in the single row, and three pairs of tubercles in the 

 double rows, but on the shorter plates they are much fewer and less regular (fig. 2 d). 

 On the under surface the tubercles are larger, and arranged in concentric rows ; there are 

 from six to eight tubercles on each plate, the smooth areolas of which nearly touch (fig. 2 b) . 



The tubercles on the upper surface of this species are extremely small, and form a 

 remarkable contrast to those in the same region of the test in Pygaster semisukatus ; their 

 areolas are scarcely sunk, and the granulations on the surface of the plates are very small, 

 and placed rather widely apart (figs. 2 a, c, e). 



The single inter-ambulacrum is concave between the disc and the border, and the anal 

 opening occupies its upper third (fig. 2 e) ; this aperture is of the same shape, but smaller 

 than in P. semisukatus (fig. 2 a) ; the portion of the area below the vent is flattened, and 

 the tumid ridges are absent. 



The base is partly concealed by firmly adhering matrix, which cannot be removed 

 without the risk of splintering the test; enough, however, is exposed, to show that 



