FROM THE UPPER GREENSAND. 203 



they are longest, to tlie apex, where they are shortest, but their vertical depth is nearly 

 the same throughout ; their surface is thickly covered with minute secondary granules 

 placed in very regular horizontal rows (fig. 4y). Each plate exhibits a more or less 

 distinct sub-central carination, so that in many specimens the inter-ambulacral areas 

 appear to be partitioned by two prominent ridges on each side of the line of junction of the 

 plates. Along the carinated ridge each plate develops a larger tubercle, and these form a 

 complete series from the base to the summit (figs. 4, a, h, d, (/) : between this and the 

 median suture there are two smaller tubercles placed horizontally (fig. 4^), and between 

 the carina and the poriferous zones are two or more similar tubercles placed in 

 two oblique series with reference to the larger central tubercles on the line of carina- 

 tion ; consequently the primary tubercles placed between the carinated ridge and the 

 median suture are horizontal, and those between the ridge and the zones arranged in 

 obhque rows (fig. 4). At the ambitus the tubercles are more numerous and less regu- 

 larly disposed. The basal plates resemble very much those on the upper surface ; in 

 full-grown specimens they are from six to eight in number; they are of the same ver- 

 tical depth as those on the dorsal surface, and are ornamented in a like manner. 



The ambulacral areas are half the width of the inter-ambulacral; the plates are 

 numerous and narrow, four plates occupying the vertical depth of one inter-ambulacral 

 plate (fig. 4 g) ; they are closely covered with small granulets, and each plate supports a 

 small primary tubercle. These tubercles are so placed that they form oblique rows of twos 

 or threes, and do not form direct vertical rows. The plates forming the ventral portion 

 of the areas are rather larger than those on the dorsal surface (fig. 4 c). 



The poriferous zones are straight and extremely narrow ; the pores are unigeminal, 

 and the pores of each pair are obliquely placed (fig. 4 //, g). Each inter-ambulacral 

 column contains twenty-one plates, and each ambulacral column eighty-four, and 

 there are one pair of pores opposite each plate; it follows that each zone contains 

 eighty-four pairs of pores ; all the primary tubercles are raised on elevated bosses in 

 areolar spaces, and they are ail perforated (figs. ^f,g, A)- 



The apical disc is small, and makes a slight prominence at the vertex ; the right 

 antero-lateral plate is the largest, and supports a large madreporiform body (fig. 4 e) ; the 

 single ovarian plate is imperforate, as in all its congeners. The ocular plates are very 

 small, and closely fitted in between the ovarials. 



The mouth is circular, and occupies a deep depression in the centre of the under 

 surface ; it is about one fourth the diameter of the base ; the peristome is feebly 

 decagonal, and the lobes are nearly equal (fig. 4 c). 



The vent occupies a considerable portion of the basal region in the single inter-ambu- 

 lacrum, between the peristome and the border ; it is of a pyriforni shape, having its small 

 extremity directed inwards towards the mouth with a series of tubercles surrounding it 

 (fig. 4 c). 



Like other Discoidea, moulds of the interior diff'er materially from the external shape 



