FROM THE WHITE CHALK. 269 



section it contains about 20 pairs of holes in each zone, and in the non-petaloidal portion 

 of this area the small pairs of pores can be distinctly traced to the ambitus and onward 

 to the mouth-opening, fig. 1 e, h. The ambulacral plates in the petaloidal portions of 

 the areas are elongated and flat in the single area, and short, convex, and tumid in the 

 antero- and postero-lateral pairs. There are several miliary granules on the plates in the 

 single arc and only solitary granules on the convex surface of those in the pairs. 



The inter-ambulacral areas are formed of very large plates (fig. 1 a, c, d, e), which 

 on the upper surface have a conspicuous mesial suture in each of the areas. All the 

 plates are covered with several horizontal rows of tubercles, arranged in a quincuncial 

 manner, fig. 1 g, on their surface, and the inter-areolar space on the same is profusely 

 covered with microscopic granules ; fig. 1 g exhibits three of the inter-ambulacral and a 

 portion of the petaloid and non-petaloid portions of the ambulacral plates, magnified 

 three diameters ; the arrangement of the tubercles and granules is likewise well delineated 

 in this beautiful drawing. The tubercles are raised on bosses encircled by sunken areolas, 

 each of which is surrounded by a circlet of microscopic granules, so that the surface 

 of the test in a well-preserved specimen, such as the type I have figured, is highly 

 ornamented. 



The posterior border is a very limited space, and is truncated obliquely downwards 

 and inwards; in its upper portion the vent is situated (fig. 1 d). The periprocte is large, 

 circular, and supra-marginal, and its position in relation to the base is very well shown 

 in fig. 1 c and fig. 1 d. 



The apical disc is small in proportion to the size of the shell. It consists of four 

 perforated ovarials and five small, heart-shaped eye-plates ; the antero -lateral ovarial 

 is much larger than the others, and extends obliquely into the centre of the disc, with 

 the spongy body covering its surface as in fig. 1 a, where the relation of the disc to the 

 petaloid ambulacra is shown, and in fig. 1 /, where the disc is drawn magnified eight 

 diameters. 



The base is nearly flat, and most accurately delineated in fig. 1 b. The course of 

 the basal portions of the postero-lateral ambulacra are observed dividing the base 

 into a central and lateral regions ; the central extends from the lower labial prolongation 

 backwards becoming merged in the posterior border, and is closely covered with 

 tubercles, close-set and arranged with great regularity in this space : the lateral regions 

 extending to the sides and anterior border have fewer tubercles, which are larger in size, 

 but more sparse in their distribution than those which occupy the central shield. I have 

 given a figure of the basal tubercles in fig. 1 h, showing the boss with its areola and 

 circle of granules, and the perforated tubercle raised on the summit of the radiated 

 bossal elevation. The enlargement is eight times, so that the comparative magnitude 

 of the tubercles on the upper and under surfaces of the test of this species may be fairly 

 estimated by comparing fig. 1 i with 1 h, and the whole in relation to the test in 

 figs. 1 a and h. 



