FROM THE CORALLINE OOLITE. 129 



ambiguum, Desmoulins, from the Terrain Jurassique Ardennes, is another name for the 

 same form, but nothing can be stated positively on this point, as Desmoulins's note on 

 this species is evidently written with much hesitation. M. Desor, in taking the name 

 Pseudodiadema to designate his genus, has retained the specific name hemispharicum for 

 Lamarck's species ; whilst M. Cotteau has figured and described a Corallian Pseudodiadema 

 under that name, which he considers to be distinct from it. 



This fine large urchin has a hemispherical form ; it is depressed above and fiat 

 below, almost entirely circular at the circumference, and rarely having a pentagonal form 

 (PI. VIII, fig. 1 a, b). The ambulacral areas have two rows of marginal tubercles, which 

 are smaller, and placed more closely together, than those of the inter-ambulacral areas ; 

 they are largest at the equator, and diminish rapidly in size in the upper parts of the 

 areas, so that, when viewed at the equator, the primary tubercles of both areas are nearly 

 of equal size (fig. 1 c), whilst above, those of the ambulacra are disproportionately small 

 when compared with those of the inter-ambulacra (fig. 1 a) ; there are twenty tubercles 

 in each row, with a zigzag line of small tubercles, extending two thirds up the areas 

 (fig. Id); the poriferous zones are straight (fig. I a, c, d) ; the pores are small and 

 unigeminal throughout, except at the base, where a few are bigeminal (fig. 1 b), and near 

 the peristome, where some are trigeminal (fig. 1 /) ; the septa between the two holes 

 forming a pair are elevated, and these elevations form a moniliform line, which extends 

 down the centre of the zone, separating the pores from each, and defining the boundaries 

 of the areas (fig. 1 d) ; there are five pairs of holes opposite each of the large inter-ambu- 

 lacral plates (fig. If/). 



The inter-ambulacral areas are more than twice the width of the ambulacral (ftg.l a,b,c,d); 

 they are furnished with two rows of primary tubercles, having fifteen or sixteen in each 

 row, which have their greatest development at the equator, and gradually diminish as they 

 approach the apical disc (fig. 1 a), and the peristome (fig. 1 b) ; the primary are flanked on 

 each side by one or more rows of secondary tubercles down the centre of the area 

 (fig. 1 c, d) ; the miliary zone consists of two rows of secondary tubercles, which separate 

 the primary rows from each other (fig. 1 d), and two rows of secondary tubercles separate 

 the primaries from the poriferous zones (fig. Id); the secondary tubercles are very 

 irregular in their size and arrangement ; they are best developed at the base and equator 

 of the test (fig. 1 b, c) ; besides the primary and secondary tubercles just described, the 

 surface of the plates is covered with numerous granulations, which form circles, more or 

 less complete, about the base of the tubercles (fig. Id). 



The tubercles of both areas, at and near the equator, are raised on prominent bosses, 

 the summits of which are all finely crenulated ; the base is surrounded by a narrow areola, 

 around which small granules and minute tubercles form more or less complete scrobicular 

 circles (fig. 1 d) ; the areolas at the equator are all confluent above and below, whilst those 

 in the upper part of the areas are nearly, if not quite, complete (fig. 1 a). 



It is remarkable how often the apical disc of this species is preserved in situ (fig. 1 a) ; 



17 



