56 CIDARIS 



The apical disc is very wide (PI. VI, fig. 1, a) ; the ovarial plates are large, thick, 

 irregular pentagons (fig. 1, y), and the ocular plates are heart-shaped; the surface of 

 both series is covered with a close-set granulation ; the ovarial and ocular holes are very 

 conspicuous in this species ; within the ovarials the external circle of anal plates are found 

 i// siiu (fig. 1, a,ff, //). 



The mouth-opening (PI. VI, fig. 1, b) is nearly circular, and smaller than the anal 

 opening; the jaws are strong and powerful, as seen in PI. VII, fig. 1, a, b. 



The spines are long, cylindrical, and fusiform, thickest at the lower third, and tapering 

 gently from thence to the point (PI. VI, figs. 2 — 5) ; they are covered with small, spiny, 

 equal-sized granules, the points of which are directed outwards ; they are arranged in 

 regular, longitudinal rows, with a valley between them ; the prickles diminish in size near 

 the collar, and at the summit of many of the large spines the rows of granules form a 

 stellate termination (PI. VI, fig. 3, a, b, c) ; the valleys are covered with a finely corriigated 

 surface, the eminences forming microscopic longitudinal lines. The neck is very short 

 and delicately striated (fig. 4, b), the head moderately large, and the milled ring not 

 prominent ; the rim of the acetabulum is feebly crenulated, and its concavity contains 

 a deep depression for the round articular ligament ; the length of the large spines, figured 

 in PI. VI, figs. 3 and 4, contained in the British Museum, is two inches and nine tenths. 

 One figured by ^I. Cotteau, from the French Chalk, is three inches long. 



Affinities and Differences. — Cidaris sceptrifera so closely resembles, in many points of 

 structure, Cidaris subvesiculosa, d'Orbigny, that by some they have been thought to be 

 varieties of one species ; the facts on which their distinctness may be maintained are the 

 following : Cidaris subvesiculosa has a more elevated test, with a greater nimiber of plates 

 in a column, the areolas are smaller and less depressed, the circle of scrobicular granules 

 is less prominent, and the obsolete tubercles on the upper plates of the columns are 

 smaller, more numerous, and differently formed ; the grannies covering the miliary zone 

 are smaller, and more regularly disposed in rows radiating from the areola. The grand 

 distinction is found in the spines, which are much smaller in C. sceptrifera, tapering, but 

 not fusiform, and covered with longitudinal serrated lines. Compare the different excellent 

 figures of Cidaris subvesiculosa, d'Orbigny, in PL VIII, with the admirable drawings of 

 Cidaris sceptrifera, Mantell, in PI. VI and VII. 



Locality and Stratigrapldcal Position. — Cidaris sceptrifera is found in the Upper 

 "White Chalk of Kent, Sussex, and Wilts. The truly magnificent specimens figured in 

 Pi. VI, fig. 1, and PI. Vllrtf, fig. I, belong to the British ]Museum; the specimens figured in 

 PI. VI, fig. 2, and PI. VII, fig. 1, belong to the Museum of the Royal School of Mines. 

 I need scarcely add they are the finest known. 



In France, M. Cotteau enumerates the following localities in which this species is 

 found : Dieppe (Seine- Inferieure) ; la Faloisc (Somme) ; Vernonnet, Giverny, Civieres, 



