40 CIDARIS. 



sides, and not much depressed at the poles (PI. I, fig. 3 a, b) ; the ambulacral areas are 

 very narrow and sinuous, having two rows of small, equal-sized, close-set granules arranged 

 on the margins thereof; the poriferous zones are rather deeply sunk, from the promi- 

 nence of the ambulacral granules, and those of the scrobicular circles, which extend to the 

 zones ; the pores are round, and the septa between the pores are about the thickness of 

 the diameter of one of the holes : there are thirteen pairs of pores opposite one inter- 

 ambulacral plate. (PL I, fig. 3 c.) 



The inter-ambulacral areas, at the equator, are five times the width of the ambulacral ; 

 each tubercular column consists of six plates, which support very large, prominent tubercles ; 

 the areolas are circular, and surrounded by a complete scrobicular circle of prominent, well- 

 spaced-out granules (fig. 3 c) ; the mammillary bosses are small ; the summits of the three 

 inferior mammas are smooth and destitute of crenulations, those of the two or three upper 

 ones are feebly crenulated ; the tubercles are disproportionately large to the size of the 

 bosses, and even of the test itself; they are hemispherical eminences terminating a stout 

 stem, and all their perforations are in the form of oblong slits ; the size and prominence 

 of the tubercles form an excellent diagnostic character for this species ; the scrobicular 

 circle consists of fourteen small, round, prominent granules, raised on little basal 

 eminences, and forming a distinct beaded wreath around each areola (fig. 3 c). 



The miliary zone, between the two rows of tubercles, is slightly concave; it does 

 not extend throughout to the apical disc, in consequence of the large upper tubercles 

 encroaching on each other ; in its widest part it is filled with three rows of granules, much 

 smaller in size than those forming the scrobicular circle. 



The mouth opening is circular, about one half the diameter of the test at the equator ; 

 the primary tubercles near the peristome are well developed, and very prominent ; they 

 increase gradually in size towards the upper surface ; the opening for the apical disc is of 

 moderate dimensions. 



The spines which are referred to this species (PI. I, fig. 3 d, e) belong to the same 

 group as that to which Cidaris florigemma belongs. They are from an inch and a 

 quarter to an inch and a half in length ; the stem swells gently outwards towards its 

 inferior third, and then tapers to the extremity ; it is covered with longitudinal rows of 

 granules, united together by lines of calcareous threads (fig. 3/), which shows the surface 

 of the spine enlarged several diameters. As these spines have not been found attached to 

 the test, it is only the smoothness of the acetabulum, and the similarity in size, that gives 

 probability to the supposition that they really belonged to the test to which we have pro- 

 visionally assigned them. As many grave errors have been committed by assigning spines 

 to tests to which they never belonged, we caution the reader, that unless spines have 

 been found on the test, or in situ on the tubercles thereof, we must treat the supposition 

 of their belonging to the same individual as a question of doubt. 



Affinities and differences. — Cidaris Wrightii resembles Cidaris projpinqua in the dis- 



