FROM THE GREAT OOLITE. 79 



zones (fig. 6 d) are narrow ; the pairs of pores are placed obliquely upwards and outwards ; 

 between each pair there is a small elevated graiuile ; these interporous granules form a 

 moniliform undulated line, which marks the course of the zones. 



The inter-ambulacral areas are very regularly formed, they are three times the 

 width of the ambulacral areas, and occupied by two rows of primary tubercles, from six to 

 seven in each row, which increase very regularly in size from the peristome to the equator, 

 and diminish in like manner as they approach the apical disc ; the areolas are large 

 and circular ; those of the five or six lower tubercles are confluent above and below, one 

 row of miliary granules forms a series of crescents, which suiTound their inner margin, 

 separating them from the poriferous zones, and two rows of the same-sized granules form a 

 narrow zigzag inter-tubercular space down the middle of the areas ; the two uppermost 

 plates of the tubercular columns have the small tubercles they develop, alone surrounded by 

 a distinct and continuous circle of scrobicular granules ; the three largest areolas are 

 channeled at their circumference ; the mammillary bosses have a wide base, and are very 

 prominent; their summits are deeply crenulated; the tubercles are moderately large ; they 

 have a short stem, and are deeply perforated. 



The apical disc (fig. 6 /) is large and prominent ; the antero-lateral genital plates are 

 much the largest; the postero-lateral are longer and narrower, and the single posterior 

 plate is the smallest and narrowest ; the right antero-lateral plate, as usual, supports a 

 conspicuous madreporiform body ; the eye plates are heart-shaped, and nearly all of the 

 same size, and the whole of the elements of the disc are covered with small close-set miliary 

 granules ; the anal opening is large, and widest in its transverse diameter. 



The base is flat, and the mouth-opening (fig. 6 b), especially in the larger specimens, 

 is very wide, being rather more than half the diameter of the test ; the lobes of the peris- 

 tome are nearly equal in size, those of the ambulacral being a little larger than those of 

 the inter-ambulacral areas. 



Affinities and differences. — Hemicidaris Luciensis very much resembles Hemicidaris 

 Wrigldii, Desor, but it is distinguished from that species by having less prominent 

 ambulacra, without the rows of granules which fill up the area in that form. Like 

 as in Hemicidaris Wrigldii, the apical disc is very prominent, and the surface of the plates 

 is covered with numerous granules. 



Hemicidaris Luciensis is distinguished from Hemicidaris Bravenderi by having less 

 prominent marginal granules in the ambulacra, a smaller and more prominent apical disc ; 

 the mouth opening is more unequally lobed ; and it is altogether a smaller and more 

 depressed form. Although the critical distinction between these Bathouian species is 

 sufficiently clear, still they have so many affinities in common, that unless the specimens 

 are well preserved, and the determination is carefully made, they may be readily mistaken 

 for each other. 



