66 CIDARIS 



The spines exhibit a considerable variation of form ; in some they are elongated and 

 cylindrical, as in PI. IX, fig. 1 ^ ; or elongated and subfusiform, as in fig. 5, and PI. X, 

 fig. 1 h, and figs. .5, 6. In all the stem is slightly enlarged in the middle, and tapers 

 towards the upper third. The surface in some specimens is sculptured with fine longi- 

 tudinal lines, as in PI. IX, fig. 1, and Pi. X, figs. 1, 4, 6; or has granulated ridges with 

 intervening valleys, as in PI. IX, fig. 5, and PI. X, fig. 5. The summit is truncated more 

 or less in all the specimens, and exhibits a stellate figure with several central convexities as 

 in PI. IX, figs. 4 a, and 4 c, 5, the radii being formed by the development of the longi- 

 tudinal ridges. 



In the more finely sculptured spines the longitudinal lines on the stem have a granuliform 

 structure; the intervening valleys are finely shagreened throughout, and provided with 

 delicate subgranular striae. The neck is short, distinctly defined, with a finely sculptured 

 line above the ring (PI. IX, figs. 4, 5, PI. X, figs. 1, 4, 6). 



The milled ring is moderately prominent, with coarser lines than those on the neck ; the 

 articular cavity is smooth, or has some feeble crcnulations on its margin (PI. IX, figs. 4, 

 5, PI. X, figs. 1, 4, 6). 



Affinities and Differences. — This form has long been considered to be a mere variety of 

 C. sceptrifera ; the proximal discal plate, liowever, has generally a rudimentary tubercle 

 of larger size and rounder shape than that found on C. sceptrifera. The second discal 

 plate (counting downv^'ards) has the upper three quarters of the boss much more strongly 

 crenulated than in the other species (at p. G4 the upper bosses of C. hirudo were 

 accidentally stated to be not crenulated). The large size of the areolas, their comparative 

 continuity, the prominent raammillated granules upon their circumference, and the 

 circumstance of the highest areola bearing a perfect tubercle being distant from the anal 

 margin by not more than half its diameter, easily separate this species from C. sceptrifera, 

 in which the areolas have more sloping borders, smaller and more numerous mam- 

 millatcd granules, and in which the highest areola bearing a perfect tubercle is generally 

 distant from the anal margin by the length of its diameter. The spines are much 

 shorter and more uniform in diameter than in C. sceptrifera, having their greatest 

 swelling midway between the acetabulum and summit, instead of towards the former ; 

 their extremities are more truncated, often becoming stellate, as on Plate IX. fig. 

 4 c; their surface is covered with longitudinal ridges, armed with very much shorter 

 piny projections, often almost obliterated ; the collar is shorter, and the acetabulum is 

 marked with stronger crcnulations. C. hirudo is a rather small species, less than 

 C. sceptrifera, and not so common. An average size wiU be about one inch and one 

 tenth in transverse diameter, height six tenths, length of spine one inch, greatest 

 diameter of spine three twentieths. 



Locality and Strafir/raphical Position. — The specimens I have examined have been 

 collected from the White Chalk of Sussex and Gravesend. In France M. Cotteau 

 gives the following localities where this Urchin is common — the Etage Cenomauien, de Havre 



s 



