FROM THE CORALLINE OOLITE. 135 



From Pseiidodiadema versipora it is distinguished by the unigeminal character of its 

 poriferous zones, the greater prominence of its bosses, the smaller size of its tubercles, the 

 greater width of the ambulacral areas, and the narrowness of its miliary zones. From 

 Pseudodiadema radiata, which has likewise unigeminal pores, by the size of the bosses 

 and the narrowness of the miliary zones, and the absence of the lateral tumidity which 

 characterises that species. From Pseudodiadema Jiemispharicum it is distinguished by the 

 uniformity in size between the ambulacral and inter-ambulacral tubercles, which are very 

 unequal in that large species ; and by the smallness of the secondary tubercles, and their 

 limitation to the zonal sides of the area ; whereas the secondary tubercles are developed on 

 both sides of the primaries in Pseudodiadema hemispharicum, a character which forms a 

 good diagnosis between these two Coralline Oolite forms. 



Locality and StratigrapJiical position. — I have collected this species in the Clays and 

 Limestones of the Coralline Oolite, at Calne, Wilts, associated with Hemicidaris intermedia, 

 Acrosalenia decorata, Echinus gyratus, and Echinobrissus scutatus. On many slabs all 

 these species are sometimes found clustered together in a more or less fragmentary con- 

 dition. It has been collected from the Coral Rag, at Redcliff, near Weymouth ; specimens 

 from this locality are in the British Museum, and Geological Museum, Jermyn Street. 



On the Continent, it has been collected by M. Roemer from the Upper Coral Rag of 

 HUdesheim, Hanover, and in the " Corallien etage de la Rochelle, de Verdun, France." — 

 Desor. It is everywhere a rare species. 



History. — First described and figured by M. Roemer under the name Cidarites 

 mamillanum ; afterwards figured and described as Diadema Davidsonii, in the ' Annals of 

 Natural History.' But a comparison of the EngHsh with the Foreign types has proved 

 them to be identical. 



