52 CIDARIS 



Test small, circular, convex above, flat below, poriferous zones narrow, depressed, 

 slightly flexed, pores oval, approximated, separated by a small granule ; ambulacral areas 

 sinuous and depressed in the middle, with from six to eight rows of small, round granules ; 

 inter-ambulacral areas wide and inflated above, with four to five small perforated tubercles 

 in each row, placed wide apart, with a miliary zone between them ; spines long, slender, 

 cvlindrical, covered with longitudinal, compressed, spiny ribs, having a wide valley and 

 uranulated surface between them. 



Dimensions. — Transverse diameter, one inch and one sixth; height, six tenths of 

 an inch. 



Description. — '^I'his beautiful urchin was distinguished by Professor Forbes from 

 C. clavigera, for which it had been mistaken by Reuss, and was first figured in Dixon's 

 ' Geology of Sussex;' it forms a well-marked species, which is very rare in our Cretaceous 

 rocks. The test is small, and circular ; inflated at the equator and upper surface, and 

 flattened below; the poriferous zones are slightly flexed, narrow, and depressed; the pores 

 are small, oval, and approximated ; there is a granule between them on the septum ; and 

 an oblong transverse ridge between each pair. The ambulacral areas are narrow, and 

 slightly undulated ; the plates incline gently towards the median suture, and are provided 

 with from four to six rows of small, close-set granules, of which the external rows are the 

 largest, and raised on mammillated eminences ; the smaller and shorter middle rows 

 disappear near the disc and the peristome, where there are only the two external rows 

 of mammillated granules ; there are from fifteen to seventeen files of granules opposite 

 one of the large plates. The wide inter-ambulacral areas are slightly inflated above and 

 near the equator, and have from four to five deep plates in each series ; the areolae are 

 small, and gradually increase in diameter from the peristome to the uppermost tubercle ; 

 on the uppermost plate of each alternate series the tubercle is nearly obsolete ; the border 

 of the areola is a little elevated, and surrounded by a circle of small, spaced-out, mammil- 

 lated granules (fig. 1, d), a little larger than those which fill the miliary zone; the boss 

 is small, the summit smooth, and the tubercle perforated in all the specimens I have 

 seen ; the inter-tubercular spaces and central miliary zone are filled with a fine, uniform, 

 close-set granulation ; on the upper part of the shell the tubercles nearest the disc are 

 small, imperfectly developed, and surrounded by the merest trace of an areola; the 

 sutures of the plates are depressed and conspicuous; the peristome (fig. \,h)vi small, 

 circular or subpentagonal ; the discal opening is larger than the peristome, and sub- 

 pentagonal ; a portion of the disc, concealed in the specimen (fig. 1, a), exhibits a finely 

 granulated surface. 



In a fossil I collected twenty years ago at Lewes, in Sussex, many of the spines 

 are preserved in situ on the fragment of the test (fig. 1, e,f,g)\ they aie long, slender, 

 and cylindrical, having seven or eight compressed ridges, with a denticulated border; the 



