«. 



FROM THE WHITE CHALK. 61 



name C. Merceyi. The test is large and circular, elevated at the upper surface, inflated at 

 the equator, and flat at the base (fig. 1, a, b). The ambulacral areas are narrow and 

 slightly flexed, much depressed at the medium suture, and furnished at the widest part 

 with six rows of granules. The two marginal rows have larger granules, very regular in 

 size and arrangement, and raised on small plates (fig. 1, c) ; the four inner zones are 

 much smaller and less regular (fig. 1, c) ; the porifsrous zones are narrow, deeply sunk, and 

 slightly bent; the small round pores are disposed in oblique pairs, of which there are 

 twenty-one, opposite one large equatorial plate (fig. 1, c). 



The inter-ambulacra] areas are wide and largely developed ; there are from six to 

 seven plates in each column, of which the four or five lower plates have large areolae, with 

 moderately sized tubercles ; the two uppermost plates are destitute of areola?, and have 

 small rudimentary tubercles (fig. 1, a and b). The areolae at the base and equator are 

 narrow and deeply depressed, well spaced out from each other at the equator, and set 

 closer together at the base. The scrobicular margin of one of the equatorial plates is 

 surrounded by a circle of eighteen regular mammillated granules, larger than those filling 

 the miliary zone (fig. 1, c) ; the boss is prominent, the summit smooth, and the tubercle 

 moderately large and widely perforated (fig. 1, d). Above the equator the two or three 

 upper tubercles entirely change their character, the areolae become extremely narrow or 

 disappear (fig. 1, b), and the tubercle becomes a mere rudiment in the midst of the miliary 

 granulation. The plate of this series nearest the equator, the third from the discal end, 

 supports a very narrow areola (fig. 1, c) with a small tubercle, and on the two uppermost 

 plates the tubercles are mere warty rudiments surrounded by granules. 



The inter-ambulacral plates are large, convex, and inflated (fig. 1, c, d, e) towards the 

 middle, and slope gently towards the sutures, which are very well marked in this species. 



The miliary zone is wide and well developed, depressed in the middle and between 

 the plates; the granules are fine, abundant, and homogeneous, and arranged in regular 

 lines that radiate horizontally from the areolae to the borders of the plate (fig. 1, c, d, e), 

 resembling in this respect C. subvesiculosa. 



The mouth-opening is small, and the peristome pentagonal ; the upper surface of the 

 test is fractured, and the disc absent. M. Cotteau, who has figured a very complete 

 specimen of this Cidaris, says that the periproct is pentagonal and star-shaped, and the disc- 

 solid and inflated, and larger than the peristome ; the ovarial plates are thick and angular, 

 their internal surface smooth and marked by three facettes for articulation with the 

 external row of anal plates ; the ocular plates are small, subpentagonal, deeper than wide, 

 and not. notched at the summit of the ambulacra. 



The spines have not been found in relation with the test. 



Affinities and differences. — This species resembles in so many important particulars 

 the preceding species, that I hitherto considered it to be a conoidal variety of that form. 

 M. Cotteau says this is one of the most curious species of Cidaris. Its form is in 



