FROM THE GREAT OOLITE. 83 



Hemicidaris Ramsayii, Wright, nov. sp. PI. VIII, fig. G a, b, c, d, e. 



Test small, circular, much depressed. Ambulacra expanded below, to enclose six large 

 semi-tubercles ; very narrow and flexuous above, with two rows of small imperforate 

 granules placed alternately on the margins thereof, and forming a single row only above. 

 Poriferous zones narrow ; pores set obliquely in pairs, with a prominent elevation of the 

 septa. Inter-ambulacra wide, with two rows of very large tubercles, four or five in each 

 row. Apical disc large and prominent ; the genital plates with a depression near their 

 centre j mouth opening large ; peristome decagonal, with unequal-sized lobes. 



Dimensions. — Height, one fifth of an inch ; transverse diameter, two fifths of an 

 inch. 



Description. — This is the smallest, but certainly not one of the least interesting of the 

 genus to which it belongs. It is remarkable for the disproportionate magnitude of three 

 of the primary inter-ambulacral tubercles to the smallness of the test that supports them ; 

 the size and prominence of the elements of the apical disc ; and the altitude being only 

 one half the latitude of the test. These three characters readily distinguish this little 

 gem from all its other congeners. 



The ambulacral areas are wide, and expanded below, to enclose from six to eight semi- 

 tubercles (PI. VIII, fig. 6 d), which are nearly as large as the inter-ambulacral tubercles in 

 the same region of the test ; they increase gradually in size, from below upwards, the two 

 superior pairs being the largest ; above the semi-tubercles the area suddenly contracts 

 (fig. 6 d), and becomes flexuous above ; on its margins there are two rows of small imper- 

 forate granules, which, from the extreme narrowness of the area above, form only a single 

 row as they approach the apical disc ; the poriferous zones are narrow ; the pores are set 

 obliquely in pairs ; and the thick septum forms a prominent granule, which separates the 

 two pores forming a pair ; there are seven pairs of pores, opposite one of the large inter- 

 ambulacral plates. 



The wide inter-ambulacral areas are almost entirely occupied by the two rows 

 of primary tubercles, which, in this species, are much larger in proportion to the size of the 

 test than in any other urchin. There are four or five plates in each inter-ambulacral 

 column ; almost the entire surface of the plate is occupied by the base of the large 

 prominent boss (fig. 6 d), the summit of which is sculptured with fine crenulations ; the 

 tubercles are very large, especially the two at the circumference, and one in each area above 

 them (fig. G b) ; on the upper part of these there is one small tubercle, near the 

 circumference of the apical disc ; a double row of small granules extends down the middle 

 of the area, which sends short branches off at right angles, by which the areas are 



