64 CIDARIS FROM THE WHITE CHALK. 



History. — The late Frederick Dixon, Esq., F.G.S., and Professor Forbes, first figured, 

 in 1850, the test and spines of this species from the White Chalk of Sussex, and the latter 

 described it as a new species under the name 0. peromelia. M. 1'Abbe Sorignet described 

 the spines which he collected in the department of the Eure under the name C. longispinosa , 

 and Professor d'Orbigny those found in the Sarthe as C. Sarthacensis. As Professor 

 Forbes first figured and described the urchin, and the other authors only described it, the 

 name of the figured specimen is for this reason retained. 



Cidauis hirudo, Sorignet, 1850. PI. X, figs. 1 — 5 ; PI. IX. 



Cidauis hirudo, Sorignet. Ours. Foss. de l'Eure, p. 17, 1850. 



— sceptrifera, Forbes, in Dixon's Geol. of Sussex, p. 338, pl.xxv, figs. 32 and 33, 



— — var. spinis truxcatis. 1850. 



— sulcata, Forbes, in Morris's Catal. of Brit. Fossils, 2nd edit., p. 75, 1854. 



— — Woodward. Mem. of Geol. Surv., Decade v, explanation of pi. v, 



p. 3, 1856. 



— hirudo, Cotteau. Paleontologie Francaise, torn, vii, p. 244, pi. 1054, 



figs. 6 — 16. 



Test, in general, of moderate size, sometimes large, slightly depressed equally at both 

 poles ; ambulacra! areas narrow, flexed, with six rows of granules at the equator, dimin- 

 ishing to two at the apertures ; in the two external rows the granules are larger and mam- 

 millated, in the inner rows they are very regularly disposed, but smaller and unequal ; 

 poriferous zones very narrow, depressed, and flexed, and formed of small round pores, 

 the intervening septum having a slight divisional elevation ; inter-ambulacral areas wide ; 

 columns with five or six: large plates; areola moderate, depressed, margin elevated, and 

 surrounded by a circle of inammillated granules, well spaced out apart ; boss with a 

 smooth summit ; tubercle moderate in size and perforated, the areola:; and tubercles in- 

 creasing gradually in magnitude from the peristome to the upper part of the columns ; 

 miliary zone depressed in the middle, and filled with equal-sized granules; line of the 

 sutures well marked throughout. 



Spines elongated, cylindrical, snbfusiform ; stem enlarged at the middle, and tapering 

 at the upper third, summit truncated and presenting a stellate figure ; the longitudinal 

 ridges on the stem have a granuliform structure, and the intervening valleys arc finely 

 ohagreened ; the neck is short, distinctly defined, and marked with longitudinal microscopic 

 lines ; the head is small, the milled ring prominent, and the acetabulum has a smooth ring 

 around the brim. 



Dimensions. — Specimen PL X, fig. 2 — height, nine tenths of an inch; transverse 

 diameter, one inch and four tenths. Specimen PI. IX — height, one inch and three 

 tenths; transverse diameter, one inch and nine tenths (?). 



