52 CIDARIS. 



forms have wide miliary zones, bat those of Cidaris Smithii are the widest ; the granules 

 of the scrobicular circle in both are not larger than those of the miliary zone, and both 

 have deeply sunk areolas. 



A comparison of figs. 1 and 2, in PL II, will show in what Cidaris Smithii differs 

 from Cidaris Jlorigemma. The latter has circular areolas, with a well-developed scrobicular 

 circle of large granules (fig. 2 g) ; narrow poriferous zones ; large tubercles, with smooth 

 bosses, and seven tubercles in each row. The spines of Cidaris fori gemma (PL II, fig. 2 d) 

 have a thick, clavate stem, with longitudinal rows of moniliform murications ; whilst 

 those of Cidaris Smithii are long, slender, and tapering, with longitudinal elevations 

 carrying forward-directed processes. 



Cidaris Smithii resembles Cidaris maxima, Munster, in the general structure of both 

 areas, but the former has more tubercles in each row in the inter-ambulacral areas 

 than the latter species. The spines of Cidaris Smithii have a more regular arrangement 

 of the longitudinal elevations and processes thereon ; the stem likewise tapers gently from 

 the neck to the apex, and wants the central swelling and irregular forward-directed 

 prickles which characterise the spines of Cidaris maxima. 



Localitg and Stratigraphical position. — This species was collected long ago, from the 

 Coral Rag of Ilillmarton, Wiltshire, by the late Dr. William Smith, the father of English 

 palaeontology. The original specimens are now in the British Museum. Mr. Lowe col- 

 lected it from the Coral Rag of Calne, Wilts. Dr. Murray, of Scarborough, found a 

 beautiful specimen in the Coralline Oolite of Ayton, near Scarborough, which is now in 

 his cabinet. The Rev. W. F. Witts collected some plates and spines in the Upper Calca- 

 reous Grit near Scarborough Castle. Mr. William Buy has found two or three very perfect 

 specimens in the Coral Rag near Calne ; and Mr. Gibbs has collected plates and spines 

 from the same formation in other localities in Wiltshire. 



This species, therefore, characterises the Coralline Oolites of England, and belongs to 

 the same horizon as Cidaris Jlorigemma, Diadema pseudo-diadema, Diadema versipora, 

 with which it is associated in the same rock. 



History. — This species has, by some, been mistaken for Cidaris jlorigemma ; and, by 

 others, has been considered to be the true Cidaris Blumenbachii. The zootomical details 

 into which I have entered show that it is very distinct from both. 



It is now figured and described for the first time. I dedicate this fine species to the 

 memory of our distinguished countryman, Dr. William Smith, whose accurate observations, 

 large views, and many researches conducted in a true philosophical spirit, led to the dis- 

 covery of that great stratigraphical law, that each of the different strata of the earth's crust 

 contains its own specific forms of organic life. 



