86 HEMICIDARIS. 



The apical disc (fig. 1/) is large ; the antero-lateral ovarial plates are the largest, and 

 the surface of the right plate is entirely covered by the madreporiform body ; the postero- 

 lateral plates are smaller, and the right is smaller than the left plate ; the odd genital plate 

 is the smallest, and the surface of all the plates is slightly roughened, with small 

 imperfectly developed miliary granules ; the genital holes are situated near the apices of the 

 plates (fig. 1/) ; the ocular plates are small, heart-shaped, and convex, and form a very 

 inconsiderable portion of the disc ; they likewise are covered with numerous small miliary 

 granules ; the anal opening (fig. 1 a, f) is large, circular, and slightly excentral. 



The test of Hemicidaris Bravenderi, on the slab (PL XI, fig. 3 a), measures one inch and 

 one fifth, and the longest spine measures three inches in length ; the head is small and 

 conical ; the milled ring is narrow, and not very prominent ; immediately above the ring 

 there is a smooth, narrow, slightly depressed neck, not broader than the thickness of the 

 milled ring ; the stem is slender in proportion to its length, and preserves a very uniform 

 diameter throughout, tapering very gently to the point; the spines of Hemicidaris 

 Bravenderi differ from those of Hemicidaris intermedia, (PI. IV, fig. 1) in the following 

 details : — the spine is longer in proportion to the diameter of the test ; the head is smaller ; 

 the milled ring is narrower, and less prominent; the neck is smooth and slightly contracted, 

 instead of having a thick prominent second ring, as in Hemicidaris intermedia; 

 (PL IV, fig. 1 n, o) the diameter of the spine is less at the base, and more uniform, tapering 

 less than the spine of Hemicidaris intermedia. In fact, the specific distinction between 

 these closely allied species is admirably shown in the spines alone, when the two specimens, 

 figured in Plate IV and Plate XI, are placed side by side. 



Affinities and differences. — It requires a minute and careful comparison of the tests to 

 distinguish the differences between Hemicidaris Bravenderi and Hemicidaris intermedia. 

 In Hemicidaris Bravenderi the ambulacral areas are straighter, the marginal tubercles 

 are smaller and fewer, being situated at a greater distance from each other ; the semi- 

 tubercles are likewise smaller, and not so conspicuous ; the primary tubercles have lower 

 bosses, less deeply crenulated at the summit, and the spinigerous tubercles are much 

 smaller; the apical disc is proportionately larger, and the inequality of size between the antero- 

 lateral and postero-lateral genital plates occasions a slight excentricity in the anal opening, 

 not observable in Hemicidaris intermedia (PL IV, fig. I a, (f); the lower part of the test 

 is likewise less inflated than in Hemicidaris intermedia (PL IV, fig. 1 c) ; it is altogether 

 a smaller form, with less prominent primary tubercles, semi-tubercles, and minute tubercles, 

 than those which adorn the shell of Hemicidaris intermedia. From Hemicidaris Luciensis 

 (PL III, fig. 6) it is distinguished by having the test more globose, with straighter and 

 wider ambulacral areas, and smaller semi-tubercles ; the apical disc is not so convex and pro- 

 minent ; the mouth opening is larger, and its peristome is likewise divided into more nearly 

 equal-sized lobes. The absence of primary tubercles from the upper parts of the inter- 



