40 CIDARIS 



The inter-ambulacral areas are wide, and consist of large, deep plates (fig. 1, c), about 

 four or five in each row, those at the equator are the largest ; the tubercle is small 

 and perforated ; the boss is flat, with a smooth summit ; the areolae are narrow and 

 complete, and encircled by a moniliform circle of sixteen small granules, raised on 

 scale-like plates (fig. 1, c) ; from the equator to the mouth the tubercles and their 

 areolae gradually diminish in diameter, and on the upper surface the plates they are very 

 deep, and tlieir areolae small, elongated, and entirely obsolete ; the inter-tubercular surface 

 of the plates is covered with close-set miliary granulations. 



The poriferous zones are narrow and deeply sunk, in consequence of the thickness of 

 the plate-ornamentation ; the holes are small, the pairs oblique, and there are sixteen 

 pairs opposite one large inter-ambulacral plate, one pair of holes being opposite one of the 

 large marginal ambulacral granules. 



The apical disc is wide, occupying all the summit of the test (fig. 1 , a, b) ; the 

 ovarial plates are large (fig. 1, d), and of an irregular rhomboidal figure; their surface is 

 covered with small granules sparsely distributed thereon, and the oviductal holes are 

 perforated at the outer third of the plates. The oculars are small and heart-shaped, and 

 intercalated between the angles formed by the ovarials ; the orbits appear to have been 

 marginal. 



Affinities and differences. — This species resembles Cidaris sceptrifera, Mant., but the 

 depth of the inter-ambulacral plates and the limited number in a column, together with 

 the smallness of the areolae and the obsolete character of those on the upper plates, prove 

 that this urchin is quite distinct from that form. 



Locality and Stratiyraphical Position. — The only specimen at present known was 

 collected by James Carter, Esq., E.G.S., of Cambridge, from the Grey Chalk of that 

 neighbourhood ; and it belongs to his collection. In Morris's ' Catalogue ' it was stated by 

 mistake to have come from the White Chalk, and in M. Desor's ' Synopsis/ from the Gres 

 vert superieure d'Angleterre. 



History. — This species was first figured in the ' Memoirs of the Geological Survey,' 

 in plate v of the Eifth ' Decade of British Organic Remains.' The original specimen 

 formed the subject of Mr. Bone's drawings for this work. 



