98 PSEUDODIADEMA 



sides and base ; they are composed of simple oval pores arranged in single file throughout 

 (fig. 3 d). The inter-ambulacral areas one half wider than the ambulacra!, have two rows of 

 primary tubercles, fourteen to fifteen in each ; a little larger at the ambitus and upper 

 surface than in the ambulacra ; three pairs are much larger at the sides, those on the 

 upper surface diminish rapidly in size between the ambitus and disc; and on the 

 under surface they are small and nearly uniform in structure. Between the basal 

 angle and the peristome there are short rows of secondary tubercles, about the size of 

 the primaries in this region, with a few scattered secondaries between the lateral 

 rows ; as all these small tubercles are nearly the same size, the under surface of the test 

 has a highly ornamented appearance — the inter-ambulacra with four, and the ambulacra 

 with two rows of small, uniform tubercles, and several secondary ones planted at every 

 interval on the plates (fig. 3 3). On the upper surface the six upper tubercles are 

 small, diminishing to mere granules around the discal opening (fig. 3 c). The large 

 ambital tubercles are smTounded by shallow circular areolas (fig. Sd). In some specimens 

 they are confluent, in others separated by one or two rows of minute granules. The 

 small dorsal tubercles are surrounded by ring-like areolas, and the basal tubercles have 

 a chain-like arrangement of granules encircling them, which adds to the ornamentation of 

 this region. The miliary zone is very large ; from the sides to the discal aperture the entire 

 surface of the plates of both areas, except those portions occupied by the areolas, is covered 

 with small, numerous, close set of granules, which form divisional partitions on each side 

 of the mesial sutures between the rows of the primaiy tubercles, and then expand 

 into a regular corrugation on all the upper surface, the dwarfing of the tubercles being 

 compensated by an increased development of granular ornamentation on this region of the 

 test. The base is very concave, and the small, ch'cular peristome, indented with well- 

 marked entailles, is situated at the bottom of a deep depression ; the entire surface of the 

 base is studded with small tubercles, surrounded with the circles of granules already 

 described. The disc is absent in all the specimens hitherto found ; the opening is large 

 and pentagonal, indicating a great development of this structure in the species. 



Affinities and Differences. — P. Hhodani is readily distinguished from its congeners by 

 the subundulated poriferous zones, small dorsal, intermediate basal, and large primary 

 ambital tubercles, by the shortness of the secondary rows limited to the base, by the 

 smallness of the dorsal tubercles and the fine homogeneous granulation on the miliary 

 zone ; the concavity of the base, smallness of the peristome, and depth at which it lies, 

 added to the highly ornamented character of the plates, form a group of characters 

 that readily distinguish it from all others. It resembles most P. Normanics (PI. XXI, 

 fig. 3), from the Grey Chalk of Folkestone, in the varied development of the tubercles 

 in each row ; the ensemble of the test in the latter form is sufficiently defined by 

 good specific characters, and for the definition of these I must refer to the article 

 on that species. 



Locality and Strati (/rapldcal Position. — The large example I have figured was found 



