FROM THE UPPER GREENSAND. 171 



Description. — This beautiful Urchin, so long known to collectors of Upper Greensand 

 fossils, has a subglobose body, depressed above, flat below, and with inflated sides ; the 

 ambulacral areas, gently flexed, stand out more prominently than the inter-ambulacrals ; 

 they have two complete rows of larger granules on the margins of the area, and two 

 incomplete rows of smaller ones within (fig. 1 c, d, and fig. 2 h) ; the poriferous zones are 

 very narrow, depressed, and much flexed, and the pores, which are unigeminal throughout 

 (fig. 1 f/), are arranged in oblicioe pairs, separated by a prominent tubercle on the septum 

 (fig. 2 h). The inter-ambulacral spaces are three times as wide as the ambulacral, with 

 two rows of tubercles, having six in each row, those above the ambitus are large and well 

 developed (fig. 1 b, d), those below are small, and decrease nuich in size as they 

 approach the peristome (fig. 2 a) ; the base acquires an ornamented appearance from 

 the number of small primary tubercles that adorn it (fig. 2 a). Each tubercle is sur- 

 rounded by a wide areola, around which a more or less complete circle of large granules 

 is regularly arranged (fig. 1 d), and most of these granules are mammillated; the miliary 

 zone is wide below, and increases in diameter above ; throughout it is covered with small 

 granules nearly of the same size (fig. 1 c andf/). At the base of the area the primary 

 tubercles and granules are disposed as in fig. 3, where a portion of the area is magnified 

 six times ; and a profile of one tubercle is given in fig. 4, magnified four times. 



The base is flat, and concave towards the centre, the mouth-opening, one third the 

 diameter of the test, is sunk in a depression ; the peristome is deeply incised, and divided 

 into ten nearly equal-sized lobes (fig. 2 a, fig. 4 (5). 



The apical disc forms a very regular structure in this species ; the antero-lateral and 

 postero-lateral ovarial plates have a rhomboidal figure, and the single or suranal plate is 

 smaller in consequence of the position of the vent ; the oviductal holes are perforated near 

 the centre of the plates ; and the madreporiform body is seen as a slight laceration on the 

 surface of the right antero-lateral in some well-preserved specimens only. The ocular 

 plates are much smaller, and transversely or subtriangularly oblong, their inner sides 

 forming the prominent portions of the triangle, and their frontal margins the bases. The 

 suranal plate occupies the centre of the disc before the vent ; it is nearly as large, and of the 

 same form as one of the ovarials, and the lines of the sutures present many punctuations. 

 At the angles of junction of every three plates is a deep punctuation, and another in the 

 line of union between every two plates. Thus there are nine punctuations around the 

 borders of the three anterior ovarials, six around each of the posterior ovarials, and three 

 around the oculars (fig. 1 e, b). The pits at the junction of three plates are triangular, 

 and those at the junction of two circular ; they are never prolonged as linear notches into 

 the substance of the plates, as in Pdlastcs dathralus and P. Bunhuryi. The vent is 

 subcircular, excentral, and inclines to the right side ; the periprocte is elevated and pro- 

 minent, and bordered by a rim formed of the elevated margins of the suranal, right 

 postero-lateral, and single ovarial plates. There is considerable variation in the size of 

 the punctuations and the width of the lines of suture ; but these variations have all their 



