116 PSEUDODIADEMA. 



tubercles nearly as large as those of the inter-ambulacra ; poriferous zones wide; pores 

 unigerainal at the equator, bigeminal on the upper and under parts of the zones ; inter- 

 ambulacral areas thrice the width of the ambulacral, with two rows of primary tubercles, 

 and a short row of secondary tubercles, one third the size of the primaries, between the 

 peristome and equator, situated between the zones and the primary row. 



Dimensions. — Height, three tenths of an inch ; transverse diameter, seventeen 

 twentieths of an inch. 



Description. — This pretty little urchin is said by Professor M'Coy, who first described 

 it, to be not uncommon in the Great Oolite of Minchinhampton, but experience has taught 

 me that it is a very rare form in the rich shelly beds of that remarkable locality. 



The test is moderately thick, and much depressed (PL VI, fig. 3 a, b) • it is distinctly 

 pentagonal, from the prominence of the ambulacra, and in its general fades very much 

 resembles Pseudodiadema depression j but the structure of the poriferous zones, and the 

 crowding together of the pores in the zones, shows it to be very distinct from that 

 Inferior Oolite form. It belongs, in fact, to that section of the genus Pseudodiadema 

 which has the pores bigeminal in the upper part of the zones (PL VI, fig. 3 c), a structure 

 which Professor M'Coy thought of generic value, and proposed the genus Dijrtopodia* for 

 the reception of these bigeminal Diadems, describing the species now under consideration 

 as a type of the same. 



The ambulacral areas are narrow and prominent, with two rows of from twelve to 

 fourteen primary tubercles, nearly as large as those of the inter-ainbulacral areas (fig. 3 d) ; 

 a few miliary granules separate the tubercles in the middle of the areas, but they disappear 

 in their upper and lower parts, in consequence of the close approximation of the tubercles 

 in these portions of the area (fig. 3 b, c). 



The poriferous zones are wide, especially above and below ; near the equator there are 

 five or six pairs of pores, which are unigeminal (fig. 3 b) ; but above these the pairs of 

 pores are bigeminal, each pair being separated by a diagonal line, formed by an elevation 

 of the surface of the zonal plates (fig. 3 c) ; between the unigeminal portion of the zones 

 and the peristome the pores are rather irregularly bigeminal, they then fall into triple, 

 oblique pairs near the mouth ; the upper part of the zones consists of small pyriform plates 

 (PL VI, fig. 3 c, d), which are neatly dovetailed together, the line of union being marked 

 by diagonal lines of elevation (fig. 3 d). 



The inter-ambulacral areas are three times the width of the ambulacral ; they have two 

 rows of primary tubercles, from ten to twelve in each row, situated on the centre of the 

 plates ; they are about the same size as the ambulacral tubercles at the equator, but are 

 much larger in proportion in the upper part of the area (fig. 3 d) ; the bosses are small, 



* 'Annals and Magazine of Natural History,' 2d series, vol.ii, p. 412. 



