70 HEMICIDARIS. 



the large equatorial tubercles are succeeded by others, which diminish suddenly in 

 size. 



The mouth opening is large ; the peristome is decagonal, its margin being always 

 divided into ten lobes by deep notches ; the five jaws, when preserved (PI. IV, fig. 1 b), 

 are always large and powerful. 



The apical disc is small, its elements are very solid, and so well articulated with 

 the areal plates, that it is very often preserved in the fossil state. It forms, in this respect, 

 a remarkable contrast to the apical disc in Cidaris, which is large, and almost always 

 absent. The anterior pair of genital plates are larger than the posterior pair, the right 

 antero-lateral plate being the largest, and always supporting a prominent spongy madre- 

 poriform body; the single posterior plate is often small and rudimentary, from the 

 encroachment of the vent ; the five ocular plates are small, prominent, heart-shaped bodies 

 with marginal orbits, whereas the oviductal holes of the genital plates are pierced at some 

 distance from the border ; the surface of all the discal elements is covered with small 

 granules. 



The primary spines are in general long, tapering, and cylindrical, but sometimes they 

 are claviform ; their surface is smooth, and sculptured with fine longitudinal lines 

 (PL IV, fig. 1 h, n, o) ; the secondary spines are small, short blunt processes (fig. 1 j, k). 



The size of the species in this genus varies from half an inch to two inches in diameter, 

 and from two tenths of an inch to an inch and three quarters in altitude. 



Hemicidaris is distinguished from Cidaris by the breadth and structure of the ambu- 

 lacra, and the presence of semi-tubercles at the bases of these areas. It is distinguished 

 from Diadema by the narrowness of the ambulacra, and the presence of minute marginal 

 tubercles only on the sides of the ambulacra, Diadema having primary tubercles as large 

 as those in the inter-ambulacra, ranging throughout these divisions of the test. 



Hemicidaris very much resembles Acrosalenia, many species of this latter genus 

 having been mistaken for the former. Hemicidaris may be readily distinguished by the 

 great difference in size between the uppermost pair of semi-tubercles and the lowest pair 

 of minute marginal tubercles, but, above all, by the size of the discal opening, and the 

 structure of the apical disc itself, which in Acrosalenia has one or more sur-anal plates ; 

 even when the elements of the disc are absent, there is always a certain amount of 

 excentricity in the opening, one angle thereof intruding farther down the single inter- 

 ambulacrum than into either of the other inter-ambulacral areas. 



Hemicidaris is distinguished from Acrocidaris by the narrowness of the ambulacra, 

 and by the semi-tubercles being limited to the bases of the areas, whereas in Acrocidaris 

 the ambulacra are wide, and furnished with primary tubercles throughout; each of the genital 

 plates likewise is furnished with a large perforated tubercle, raised on a crenulated boss. 



The genus Hemicidaris may be subdivided into three sections, of each of which we 

 have typical examples in our Oolitic formations : 



