68 CIDARIS 



CiDAEis PLEKACANTHA, B'Orhigny. Prodrome de Pal. Strat., t. ii, p. 274, fit. 22, 1850. 



— — Dixon. Geology of Sussex, tab. xxiv, fig. 23, 1850. 



— — Desor. Synopsis des fichinides foss., t. vi, fig. 7 — 10, 



p. 14, 1855. 



— — Woodward. Mem. Geo). Surv. Decade V, Expl., pi. v, 



p. S, 1856. 



— — Cotteau. Paleontologie Franc., Ter. Cret., torn, vii, tab. 1075, 



fig. 1 — 1,3, p. 310, 1865. 



Test unknown. 



Description. — Spine very large, inflated, pyriforra, with an obtuse and unequally 

 rounded summit ; stem ornamented with longitudinal striae, very fine or subgranular, and 

 visible near the lower part, the upper part is smooth ; the stem suddenly contracts to form 

 a very short neck and a small head ; milled ring a little elevated and marked by fine lines ; 

 acetabulum small with a smooth ring. 



Spines of this species are very rare indeed in the English Cretaceous rocks. The 

 specimens collected at Meudon, near Paris, and at Civieres (Eure), vary much in form 

 and dimensions ; some are short, thick, or pyriform, and have the stem round or depressed 

 at the summit, or inflated, subcylindrical, accuminated, or truncated ; and in a large 

 specimen before me from Erance the stem is bifurcated. 



Locality and Stratigrapldcal Position. — Mr. Dixon's specimens were said to have been 

 found in the Grey Chalk of Sussex. It occurs also in the Lower Chalk of Dorking. The 

 specimen figured, Plate XII, fig. 5, in the Cabinet of J. R. Capron, Esq., F.G.S., came 

 from that locality. 



CiDARis Farringdonensis, Wfiglit. PI. II, figs. G, 7, and 8 a, b, c. 



Test known only by isolated plates. 



Description. — Spines long, slender; lower portion of the stem smooth, upper portion 

 ornamented with longitudinal rows of granules forming tuberculated lines or ridges in 

 diff"erent spines, and terminating in a star-shaped summit at the apex. The proportional 

 length of the smooth to the granulated ornamentation of the stem varies in different spines 

 — in some with a long smooth portion the line of separation is defined by an annular 

 elevation, in others with a shorter smooth portion the granulations arise without any such 

 ridge. The valleys between the longitudinal ridges have a finely shagreened surface ; the 

 head is moderate in size, the milled ring prominent, and the small acetabulum has a 

 ■well-defined marginal rim. 



The isolated plates of the test are much worn by friction ; the primary tubercle is 

 small, the areola wide and smooth, and the margin surrounded by a circle of large well- 

 defined granules, resembling the plates of Cidaris vesiculosa. 



Affinities and Differences. — The spines of Cidaris Farringdonensis differ so much from 



