104 PSEUDODIADEMA 



Descri])tion. — This beautiful Urchin when full grown is moderately large ; it has a 

 circular, elevated, and inflated form, depressed on the upper surface, and concave beneath. 



The ambulacral areas are large, a little inflated, and furnished with two rows''of 

 moderately sized tubercles, 14 — 16 in a row; one of these is always longer than the 

 other, and separated from its fellow by a double series of small compact granules. The 

 poriferous zones are straight and narrow at the base of the test, and gently subun- 

 dulated from the ambitus to the apical disc ; they are composed of simple pores closely 

 approximated to each other ; the pairs are well spaced out in the infra-marginal region, 

 closer together at the equator, and still closer placed near the disc. PI. XXI, figs. 1 b, c. 



The inter-ambulacral areas are large, and provided with two rows of tubercles, nearly 

 identical in size with those of the ambulacral areas at the base and ambitus of the 

 same regions ; in the upper part of the area, however, they are proportionally larger, 

 so that we find only 13 — 15 tubercles in each row. The secondary tubercles form a 

 distinct series of from seven to nine small tubercles, situated between the primary row 

 and the poriferous zones ; in general one small tubercle arises from each plate between 

 the peristome and ambitus (PI. XIX, fig. 1 (/), and is absent from the three or four upper 

 plates of the test (PI. XVI, fig. 4 e) ; besides these, there are other smaller tubercles, of 

 unequal size, scattered between the primary and secondary rows (PI. XIX, figs. 1 b, c, d). 



The miliary zone is very wide; it is narrow near the peristome, enlarged at the 

 ambitus, and nude and depressed on the upper surface (PI. XVI, figs. 4 c and e). The 

 granules are numerous, unequal, sometimes mammillated, and distinguished with difficulty 

 from the small secondary tubercles among which they are interspersed (PI. XXI, figs. \ a,c; 

 PI. XVI, figs. 4 c, ^). 



The primary tubercles of both areas have well-defined circular areolas, prominent 

 bosses, with sharply crenulated summits, and large mammillons with deeply drilled summits ; 

 the inter-ambulacral are a little larger than those of the ambulacral areas. The areolas near 

 the ambitus are mostly confluent above and below, whilst on the upper surface of the inter- 

 ambulacra they are distinct, and surrounded by circles of small granules (PL XXI, fig. 1 c). 



The under surface is convex at the circumference, with a deep depression in the 

 middle, in which the mouth-opening is situated. The peristome is very small, and its 

 margin feebly indented. The discal opening is large, elongated, and sub-pentagonal. 

 Unfortunately, all the specimens hitherto found want the discal plates (PI. XIX, fig. 1 b). 



Affinities and Differences. — This Urchin resembles P. Michel i iii ; it is, however, distin- 

 guished from that species by its much greater height, inflated sides, narrow base, 

 slightly undulated poriferous zones, more numerous and better developed secondary 

 tubercles, more deeply sunk peristome, and elongated discal opening; these diagnostic 

 characters are not the result of age, as they are observed more or less in comparing 

 young specimens of both species apparently of the same age with each other. 



Localifi/ and Stratigraphical Position. — The specimens figured, from the Grey 

 Chalk, Folkestone, on Pis. XVI, XIX, and XXI a, belong to the Rev. T. Wiltshire, the 



