FROM THE GREAT OOLITE. 85 



tubercles, eight in each row, gradually decreasing in size from the equator to both poles ; 

 apical disc large, anal opening excentral behind ; mouth opening large ; peristome divided 

 into ten nearly equal sized lobes. 



Dimensions. — Height, nine tenths of an inch ; transverse diameter, one inch and one 

 fifth of an inch. 



Description. — It is exceedingly difficult to detect on the test alone of many allied 

 species of Hemicidaris, characters sufficiently well marked to distinguish them from 

 each other. When these forms, however, are found with their spines, the distinction 

 is in general so evident, that the difficulty at once disappears ; but when deprived 

 of these appendages, the diagnosis becomes obscure. This is well exemplified in several 

 species, and strikingly so in the one now under consideration. At the first glance 

 Hemicidaris Bravenderi would be taken by most persons for Hemicidaris inter- 

 media ; but the details of its structure afford sufficient evidence of its distinctness 

 from that Corallian type. The test is sub-globular, not much inflated at the base ; the 

 ambulacral areas are nearly straight, being only slightly undulated in the upper third ; on 

 the margin of the areas there are two rows of small perforated tubercles, from fifteen to 

 sixteen in each row ; raised on small bosses, and placed rather widely apart ; two rows of 

 microscopic miliary granules extend down the centre of the areas, and lateral branches form 

 circlets around the areolas (fig. 1 c) ; the semi-tubercles are not large, but are regular as to 

 form, size, and arrangement, the six pairs gradually decreasing in magnitude from the 

 upper or largest pair, to the smallest or most inferior pair, which extend to the margin of 

 the ambulacral lobe (fig. ] b). 



The inter-ambulacral areas (fig. 1 b), are nearly three and a half times the width of the 

 ambulacral areas ; their two rows of primary tubercles decrease gradually in size 

 from the equator, where they are largest, to both poles ; there are seven tubercles in each 

 row ; the bosses (fig. 1 b, c, d) are prominent, and their summits have from fourteen to 

 sixteen crenulations, not, however, deeply marked ; the spinigerous tubercle is small, and 

 finely perforated ; the areolas are wide and confluent (fig. 1 c) above and below ; down 

 the centre of the area (fig. 1 a, b) a zig-zag double row of small tubercles, slightly 

 perforated, and raised on little eminences (fig. 1 c), separates the two rows of primary 

 tubercles from each other ; small miliary granules fill up the interspaces at the base of 

 these elevations ; a single row of the same sized small, perforated tubercles separates the 

 areolas from the poriferous zones (fig. 1 c) ; among these, likewise, a few miliary granules 

 are irregularly distributed. 



The base is flat (fig. 1 b), and nearly two thirds of the whole is occupied by the mouth 

 opening, which is more than one half the diameter of the shell at the equator ; the 

 peristome is deeply notched, and divided into ten nearly equal sized lobes (fig. 1 e), and 

 the margin of the notches is reflexed. 



