28 CIDARIS. 



the semi-circlet surrounding the outer and inner margin of the areolas is composed 

 of granules very little larger than those which fill up the mihary zone. The primary 

 tubercles are of moderate size ; the number in each column is uncertain ; but, judging 

 from the eight tubercular plates still remaining in an imperfect column, and estimating the 

 number necessary to complete the same, it is probable that the perfect test had from 

 twelve to fourteen tubercles in each row, gradually increasing in size from the peristome 

 upwards ; they are proportionately small when compared with the size of the test itself. 

 The areolas are oblong, and the narrowness of the plates causes them to open into each 

 other above and below ; the bosses are neither large nor prominent, but the crenulations 

 on their summits (PI. I, fig. 1 b) form a well-marked circlet of granules, which surrounds 

 the base of the tubercles (fig. 1 c) ; these are small, with deep and wide perforations on 

 their summits. The peristome is wide, and, in the only three specimens we know, part 

 of the dental apparatus is preserved in situ : the portion remaining in the figured specimen 

 consists of three prominent jaws (fig, 1 a) ; one of these is shown in fig. ly. It is convex, 

 and the external surface is strengthened by three prominent ridges ; the teeth are large, 

 but are all fractured. As the test rests on its upper surface, the ovarial plates and dorsal 

 surface are concealed from observation. 



The spines remaining in situ on the test are of two kinds (fig. 1 a) ; those articulated 

 with the primary tubercles, the primaries ; and those articulated with the smaller tubercles, 

 the secondaries. The primary spines exhibit a peculiar structure (fig. If) ; the head is 

 large, gradually increasing in diameter from the articular cavity to the circular band; 

 the rim of the acetabulum is coarsely and deeply crenulated, and the raised band is nar- 

 row, prominent, and finely milled ; the neck tapers gradually from the baud to the point 

 where it joins the stem, which has the same structure as the head ; and the surface is 

 delicately sculptured with fine longitudinal lines (fig. If). 



The stem is united to the neck by an oblique harmonia suture ; in those spines which 

 are denuded of their external layer, it has a horny semi-transparent aspect ; but when 

 this external layer is present, its surface is seen, with a pocket lens, to be sculptured with 

 fine longitudinal lines ; besides these, there are a number of small processes arranged in 

 rows, the points of which have a direction forwards, but their linear arrangement is by no 

 means regular ; the stem is long, slender, and circular, its length in proportion to the 

 diameter of the test has not been ascertained, as all the spines are fractured. The 

 secondary spines are very uniform in size and structure, and are preserved in abundance 

 in the specimen before us (fig. 1 a). Each of the minute tubercles of the ambulacral 

 areas, and of the miliary zones, carried one of these spines, which measure about -/o-ths to 

 •jL-ths of an inch in length ; they are stout and blunt pointed, tapering very little from 

 the milled band to the apex, and having their surface covered with microscopic longitudinal 

 lines (fig. \ d, e). 



AJinities and differences. — This Cidaris belongs to the same group as Cidaris Fowleri, 



