FROM THE UPPER GREENSAND. 217 



hexangular outline. They vary much in the degree of height and tumidity, as compared 

 with their length and breadth ; the upper surface is sometimes convex and sometimes 

 depressed ; the anterior half of the shell is more enlarged than the posterior, the widest 

 portion being the region across the antero-lateral ambulacra (fig. 2 b, d). The obscure 

 angles, when they are five, correspond to the ambulacral areas ; when there is a sixth it 

 is in the centre of the sinde inter-ambulacrum. The sides are so rounded that their 

 most tumid portions are central or sub-central. 



In the large specimen (fig. 2 h) the base is flat, inclining to concave, in the smaller 

 specimens from the Chloritic Marl, as in fig. 3, the border is so rounded off at the sides 

 that it becomes slightly convex, in the still smaller varieties it is mostly flat. 



The ambulacral areas are narrow, tapering towards the disc and peristome, and widest 

 above the ambitus (fig. 2 a, b, c) ; they are formed of narrow plates four or five of which are 

 opposite one inter-ambulacral ; four rows of tubercles set in a zig-zag arrangement occupy 

 the area (fig. 2 e), and those at the base are much larger than those on the sides. The 

 poriferous zones are linear and the very small pores arc set in triple oblique pairs, 

 six or eight pair being opposite the vertical height of one inter-ambulacral plate. 



The inter-ambulacral areas, four times the width of the ambulacral, are formed of broad 

 and well-developed plates (fig. 2 a, b, c) ; in the large specimen (fig. 2 c), twelve of these 

 are found in each column between the disc and border, and six between the border and 

 peristome ; each plate supports two rows of tubercles set in irregular horizontal lines 

 above each other, four or five tubercles occupying each row. Fig. 2 e shows two 

 complete and two incomplete inter-ambulacral plates, and a portion of the ambulacral 

 area and poriferous zones magnified four times ; besides the primary tubercles the surface 

 of the plates of both areas is covered with numerous microscopic granules. The basal 

 have larger tubercles than the lateral plates. Fig. 2/ shows two of these plates from the 

 middle of the base magnified four times ; the crenulations of the boss and the perforations 

 at the summit are better seen in this than in the other figure. 



The apical disc is composed of five genital and five ocular plates ; four of the former 

 are perforated and the single plate is imperforate (fig. 2y). The right antero-lateral 

 genital plate is much the largest and supports the madreporiform body, which extends 

 backwards and fills the central portion of the disc. The small cordate perforated oculars 

 are wedged into the angles formed by the ovarials and complete the circle of the disc, the 

 elements of which are well soldered together to form a compact structure ; and the surface 

 of the plates uncovered by the spongy body is studded with close-set microscopic granules. 



The mouth-opening is small and oval (fig. 2 b), the long diameter extending obliquely 

 across the base from the left to the right side ; the peristome is obscurely decagonal and 

 divided into ten nearly equal-sized lobes. The base is sometimes flat, or slightly con- 

 cave, as in the large specimen (fig. 2 b), or it is flattened in the centre and rounded off at 

 the sides, or sometimes it is slightly convex (fig. 3) ; in either case the mouth-opening 

 is nearly in the middle. 



28 



