132 PSEUDODIADEMA. 



(fig. 3/) ; between the areola and the pedal pores there is a narrow lateral miliary zone, 

 formed of two rows of small granules, among which two or three small secondary 

 tubercles are interspersed (fig. 3/) ; the central miliary zone is wide; on each plate there 

 are two rows of small, close-set granules (fig. 3/), making four rows of granules at the 

 equator ; a little way above that point they bifurcate, and on the upper part of the area 

 a few scattered granules only are observed, the surface of the plates being almost naked 

 (fig. 3 d). Like the ambulacral tubercles, those of the inter-ambulacra diminish very 

 gradually in size towards both ends of the area. 



The base is slightly concave, from the tumidity of the sides of the test ; the mouth 

 opening is small (fig. 3 b, e), four tenths of an inch in diameter ; the peristome is slightly 

 notched, the ambulacral lobes being larger than the inter-ambulacral (fig. 3 e). 



The disc opening is nearly round, but the plates are unfortunately absent. 



Affinities and differences. — This species resembles Pseudodiadema mamillanum, Roemer, 

 in having a few small secondary tubercles near the base, between the poriferous zones and 

 the primary tubercles, and in having the pores unigeminal ; but it is readily distinguished 

 from that fine species by having smaller tubercles, much less prominent bosses, and a 

 wider miliary zone ; the sides of the test are likewise much more tumid, the mouth 

 opening is smaller, and the peristome is more equally lobed. 



From Pseudodiadema versipora, which occurs with it in the same rock, it is readily 

 distinguished by the unigeminal character of its poriferous zones, and the absence of 

 large secondary tubercles from the inter-ambulacral plates (PI. VII, fig. 4/). 



Locality and Stratigraphical position. — This is a very rare species ; it was collected 

 from the Coral Rag, near Steeple Ashton, Wilts, associated with Pseudodiadema versipora. 



Pseudodiadema mamillanum, Roemer. PI. VIII, fig. 2 a, b, c, d ; PI. XII, fig. 9. 



Cidarites mamillanum. Roemer, die Versteinerungen des Norddeutschen Oolithen- 



Gebirges, t. 2, fig. 1 a, b, c, p. 26. 



Diadema mamillanum. Agassiz and Desor, Catalogue raisonne Annales des Sciences 



Naturelles, 3 me serie, Zool., torn, vi, p. 347. 



— spinososum. Agassiz, Catalogus Systematicus, p. 8. 



— Davidsonii. Wright, Annals and Magazine of Natural History, 2d series, 



vol. xiii, p. 170, pi. 13, fig. 2 a — e. 



— mammillatum. D'Orbigny, Prodrome de Paleontologie, tome ii, p. 27, 



14 e etage Corallien. 

 Pseudodiadema mamillanum. Desor, Synopsis des Echinides Fossiles, p. 64. 

 Diadema mamillanum. Woodward, Memoirs of the Geological Survey, Decade V. 



Notes on British Fossil Diademas. 



