2 



BRITISH FOSSILS. 



representations of it are those given by Goldfuss and by Desor. The 

 figures in Woodward's " Geology of Norfolk," referred to in the syno- 

 nyms, are slight but characteristic. 



Description. — The outline is orbicular ; the ventral surface flat ; the 

 dorsal very convex, varying in stages of growth from regularly hemi- 

 spherical to liemispherico-cylindrical, the latter shape being its final 

 condition. The upper surface is sparingly granulated with minute 

 tubercles, and is divided into 10 narrow and 10 broad segments, those 

 of each set being of equal dimensions, by the poriferous avenues. The 

 under surface is nearly flat, similarly divided, impressed in the centre 

 for the mouth, and perforated between the mouth and posterior margin 

 for an elliptical rather small anus. 



The interambulacral segments are, in their widest part, three times 

 the breadth of the ambulacral. The plates of the dorsal surface, towards 

 the centre and lower parts of the sides, are horizontal, broad, and verti- 

 cally narrow, those only near the summit being of equal dimensions 

 everyway. Whatever be their horizontal diameter, their vertical mea- 

 surement is nearly alike in all. Their surfaces are studded with 

 minute granules^ somewhat scattered towards their sides, but clustered 

 towards their centres, where they group themselves moniliformly around 

 the few scattered small primary tubercles, of which there is only one on 

 the uppermost plates, though they gradually increase in numbers down 

 the sides to as many as seven or eight. Of these one is always on each 

 plate in the line of slight carination, which runs down each half of each 

 interambulacral area. The lowest series of plates, towards the margin, 

 bear numerous primary tubercles, of which those towards the centro- 

 ambulacral suture are arranged in regular horizontal rows. The inter- 

 ambulacral plates of the ventral surface are still more conspicuously 

 ornamented with rows of primary tubercles, each surrounded by a 

 conspicuous impressed circle, bounded by granules, which are larger 

 and more thickly set than on the upper surface. The inferior interam- 

 bulacral plates, towards the margin, are much narrower than those near 

 the mouth. The inferior interambulacral spaces bulge out on each 

 half near the margin ; a bulging which is continuous with their carina- 

 tion above, and which indicates the position of the strengthening ribs 

 in the interior of the test. 



The superior ambulacral plates are very numerous, and vertically 

 very narrow. Their surfaces are speckled with granules, and at inter- 

 vals, generally of threes, the uppermost ones bear primary tubercles, 

 which become more numerous towards the centre and margin. Each 

 plate corresponds to a pair of pores in the ambulacral avenue, which is 

 scarcely, if at all, impressed. The pairs of pores are larger and more 

 conspicuous on the dorsal than on the ventral surface. Dorsally four of 



