FROM THE WHITE CHALK. 49 



Cidaris clavigera, Pictet. Traite de Paleontol., 2nd ed., t. iv, p. 254, 



pi. xcviii, fig. 8, 1857. 



— — Cotteau. Paleontologie Francaise ; Ter. Cretace, t. vii, 



pi. 1069, 1070, 1071, p. 285, 1862. 



— — Dujardin et Hape. Hist. Nat. des Zooph. ; Ecliino- 



dermee, p. 480, 1862. 



— Heberti, Dujardin et Hupe. Ibid. 



Test circular, moderately inflated, of medium size, and nearly equally depressed at 

 both poles. Ambulacral areas narrow, slightly flexed, with four rows of mammillated 

 granules, the inner being smaller than the outer rows ; inter-ambulacral areas wide, 

 five very large tubercles in each row; areolae deep, circular, and complete; margin 

 surrounded by a circle of mammillated granules; tubercles large and imperforate, boss 

 smooth ; miliary zone wide, not depressed, furnished with large granules ; peristome 

 small, subpentagonal ; apical disc large, subcircular ; spines large, clavate, crowded with 

 longitudinal, prickly ribs. 



Dimensions. — Transverse diameter, one inch and three tenths ; height, seven tenths 

 of an inch. 



Description. — The finest examples of this urchin extant are those now figured in 

 Plates IV and V, the one belonging to my friend, Dr. J. S.' Bowerbank, PI. IV, the 

 other to the British Museum, PI. V, fig. 1 ; both these specimens were collected and 

 developed by the late Mr. Taylor, and formed part of his collection of Cretaceous Echino- 

 dermata. Having had abundant materials at my disposal for the illustration of this species, 

 I have endeavoured to give accurate figures of all parts of the test, as well as of the 

 remarkable spines which armed it. An examination of the series I have figured will 

 show how very much the spines, from different parts of the test, differ from each, and how 

 cautious we ought to be in founding species of Cidaris on the spines alone. 



The test is of medium size, circular, moderately inflated, and nearly equally depressed 

 at both poles. The ambulacral areas are narrow, slightly flexed, and provided with four 

 rows of granules ; the external are the largest, and most regular, and are elevated on 

 mammillated summits close to the border of the poriferous zones; the internal rows are 

 composed of smaller granules, irregular in size, arrangement, and extent ; towards the 

 ends of the areas they diminish in size, and gradually disappear whilst the external 

 rows are persistent throughout. A number of microscopic granules are irregularly 

 scattered among the internal rows, and form a regular series at the angle of the external 

 granules, on the border of the poriferous zones, and opposite each pair of holes. 



The poriferous zones are narrow, depressed, and slightly flexed ; the pores are large, 

 round, and open, closely approximated to each other, and having small elevated granules 

 on the septa (PI. IV, fig. 1, b). 



7 



