FROM THE WHITE CHALK. 63 



nearly straight, and provided with six rows of small granules ; the marginal series are the 

 largest, the inner ones are small, and between them are some still smaller granulets 

 scattered about the area. The poriferous zones are only gently waved, the holes round, 

 and placed transversely; there are fifteen pairs opposite each of the equatorial plates. 

 The inter-ambulacral areas are very regularly formed ; the plates, about eight in each 

 column, are large and uniform ; the areolae are wide and circular, and occupy the entire 

 depth of the plate ; the margin is surrounded by a circle of very small mammillated 

 granules, about twenty-one in number, and well spaced out from each other ; the boss is 

 not prominent, and only some of the summits are feebly crenulated, whilst the others are 

 smooth. The tubercle is small, and deeply perforated (fig. 2, a, b). The miliary zone is 

 wide, and depressed in the middle ; the granules are fine, homogeneous, and nearly equal- 

 sized, and arranged in regular horizontal rows, which follow the angles of the median 

 sutures. As the areolae occupy the centre of the plates, there is a considerable granular 

 space between the ambulacral side of the areolae and the poriferous zones. The sutures 

 are all very distinctly marked, and the median inter-ambulacral is much depressed. The 

 peristome is sub-pentagonal and large. The spines are long, cylindrical, and sub-acumi- 

 nated at the summit (fig. 4, a). The stems are provided with long, narrow spines, projecting 

 at intervals from many regular, longitudinal ridges, and having the interstices finely 

 granulated (fig. 4, c). For the most part these spines are equally and uniformly disposed ; 

 sometimes, however, they lose their homogeneity, and vary in their height and in pre- 

 serving a longitudinal disposition. The valleys between the ridges on the stem are 

 throughout covered with fine, delicate, microscopic, sub-granular, longitudinal striae, which 

 are only visible by the aid of a lens (fig. 4, c). The neck is without ridges and spines; 

 the collar is long and finely striated, and separated from the stem by a distinct line (fig. 

 4, b). The head is large, the milled ring prominent, with thicker striae than those on the 

 collar, and the rim of the acetabulum is smooth (fig. 4, b). 



Affinities and differences. — The melon-shaped test, very regular inter-ambulacral areas, 

 areolae and tubercles gradually increasing from the base to the upper surface, added to the 

 long, slender spines, with prickly ridges, serve to distinguish this species from its congeners 

 of the White Chalk. The spines resemble those attributed to C. spinigera, Cott., of the 

 Neocomian stage, from the middle of France, but they manifest differences which are 

 sufficiently distinctive of each. In C. perornata the spines arise from ridges at regular 

 intervals, which are absent in C. spinigera. 



Locality and Stratigraphical position. — This species was collected from the White 

 Chalk of Kent and Sussex. The specimens figured are from the cabinets of Dr. Bower- 

 bank and Rev. T. Wiltshire. 



The foreign localities of this urchin are, according to M. Cotteau, Vernonnet, 

 Giverney, Pinterville, Houguemarre (Eure), Epagny (Somme), Tartigny (Oise), La Fleche, 

 (Sarthe) ; where the spines are common in the Etage Scnonien. 



