118 PSEUDODIADEMA. 



limited to the Oolites ; but it has been ascertained that the same diplopodous structure of 

 the poriferous zones is found to prevail likewise in several Cretaceous species, as well as in 

 those cited by this author. Neither Prof. Agassiz nor the late Prof. Edward Porbes 

 considered the diplopodous character of the poriferous zones alone of generic importance ; 

 but Professor M'Coy and M. Desor think otherwise, the latter author having retained the 

 genus Diplopodia in his ' Synopsis des Echinides Eossiles.' I am indebted to Professor 

 Sedgwick for the loan of the type specimen belonging to the Cambridge Museum, which 

 is now figured (PI. VI, fig. 3 a, b) for the first time. 



PsEUDODiADEMA HOMOSTiGMA, Agassiz. PI. VI, fig. 5 a, b, c, d, e,f. 



DiADEMA HOMOSTiGMA. Echinodermes Fossiles de la Suisse, partii, t. 1 7, figs. 1 — 6, 



p. 24. 



— — Agassiz and Desor, Catalogue raisonne Echinides, Annales 



des Sciences Naturelles, y serie, tome vi, p. 347- 



— jEQUAie. Quenstedt, Petrefactenkunde, t. 49, fig. 29, p. 579? 

 PsEUDODiADEMA HOMOSTIGMA. Dcsor, Synopsis des Echinides Fossiles, p. 65. 

 DiADEMA HOMOSTIGMA. Woodward, Memoirs of the Geological Survey, Decade V, 



Notes on British FossU Diademas. 



Test small, depressed, nearly circular ; ambulacral areas nearly as wide as the inter- 

 ambulacral ; primary tubercles in both, nearly of the same size ; no secondary tubercles ; 

 poriferous zones unigeminal ; mouth opening large, peristome unequally decagonal. 



Dimensions. — Height, one fifth of an inch ; transverse diameter, half an inch. 



Description. — This beautiful little species was first discovered by M. Nicolet in the 

 Inferior Oolite of Chaux-de-Fonds, and figured and described by M. Agassiz in his 

 ' Echinodermes Eossiles de la Suisse/ who, in describing it, observes, " the species which 

 I have figured under this name offers none of those prominent traits which we recognise 

 in so many species. It also presents a uniformity almost hopeless for description. Up to 

 the present (1840) I only know the specimen I have figured, which is all the more valuable 

 on account of the rock (Inferior Oolite) from whence it was collected, for we know in what 

 a bad state of preservation the most of the fossils of this terrain are found in." It is 

 probable that the small Diadema aquale, figured by Quenstedt from the Brown Jura S 

 of Spaichingen, is identical with Pseudodiadema homostigma, as it has been collected 

 from the same geological horizon, and wants the rows of secondary tubercles, which 

 characterise that Coral Rag form. 



The ambulacral areas are nearly as wide as the inter-ambulacral areas, and support 



