36 CIDARIS 



and Tertiary epochs. In his valuable Synopsis, M. Desor describes six genera in this 

 family : these are Cidaris, Klein ; Mabdocidaris, Desor ; Diplocidaris, Desor ; Porocidaris, 

 Desor; Goniocidaris, Desor; Palaocidaris, Desor. Of this number three are extinct — 

 Diplocidaris, Porocidaris, and Palaocidaris ; two contain both extinct and living forms 

 — Cidaris and Eabdocidaris ; and one is only found living — Goniocidaris. 



A. — Species from the Gault. 



Cidaris Gaultina, Forbes, MS. PI. I, fig. 2, a, b, c, d, e ; fig. 3, a, b; fig. 4 a, b, c. 



Cidaris Gaultina, Forbes. Morris's Catalogue of British Fossils, 2nd ed., p. 74, 

 1854. 

 — — Woodward. Memoirs of the Geological Survey, Decade v, expl., 



pi. v, 1856. 



The specimen figured in PI. I, fig. 2, belongs to the British Museum, and was kindly 

 communicated by my friend, S. P. Woodward, Esq., F.G.S. It consists of five inter- 

 ambulacral plates, and a trace of an ambulacral area, with a number of spines. Detached plates 

 of this urchin resemble those of Cidaris vesiculosa, Goldf. In the large plate (fig. 2, b), 

 magnified two and a half times, the depth is greater than the breadth ; the wide circular 

 areola is nearer the lower border, its margin is elevated, and surrounded by a circle of thirteen 

 small tubercles, each set upon a distinct base ; the boss is flat, its summit smooth, and 

 the tubercle large and distinctly perforated ; the rest of the plate is covered with a fine 

 close-set granulation (fig. 2, a, b). 



The remaining fragment of the ambulacral area is too imperfect for description. 



The spines vary in form ; they are in general long and slender, and taper slightly 

 (figs. 2 and 3). The surface is covered with longitudinal ridges, the edges are distinctly 

 serrated (fig. 2, a, and fig. 3, b), and the stem represents a miniature fluted column. The 

 truncated summit shows a star-like structure (fig. 2, e), produced by the serrated ridges 

 meeting around the circumference of a central circle. The neck is smooth, short, and 

 thick (fig. 2, d, and fig. 3, b) ; the milled ring broad and flat, and covered with very fine 

 lines ; the head is short, and the acetabulum surrounded by a line (fig. 2, d). 



In one of the broken spines that accompanies the plates the upper extremity is 

 expanded and cup-like (fig. 4, a, b, c), whilst in the other spines the extremities are 

 contracted and truncated (fig. 2, a, c). 



Affinities and differences. — This urchin very closely resembles Cidaris vesiculosa, Goldf., 

 found in the Chalk-marl of Essen and Ruhr. The resemblance is so great between the 

 German and English forms, that without a comparison of specimens a correct diagnosis 



