ECHINODERMATA. 49 



poriferous zones are likewise very much narrower, and the pores are smaller and set closer 

 together; the inter-ambulacral areas are wider, whilst the miliary zone is narrower; 

 the scrobicular circles are likewise much more prominent, and have larger granules, 

 raised on distinct eminences ; the smooth summits of the mammillary bosses of the 

 lower tubercles, and the faint crenulations on those of the upper ones, form a striking 

 contrast to the broad, deeply crenulated summits of the mammillary bosses in Cidaris 

 Fowleri. 



The same group of characters which serve to distinguish Cidaris florigenma from 

 Cidaris Fowleri form the diagnosis between it and Cidaris Orhignyana. 



Cidaris florigemma resembles Cidaris SmitJiii, Wright, with which it is occasionally 

 associated in the same rock in Wiltshire and Yorkshire, in the general form and height of 

 the test, but it is distinguished from this much rarer species by having fewer tubercles in the 

 inter-ambulacral areas, and much more prominent scrobicular circles around them ; Cidaris 

 Smitliii having ten primary tubercles in each column, and the granules of the scrobicular 

 circles not being much larger than those filling the miliary zone ; the ambulacral areas 

 are likewise narrower, and the poriferous avenues are narrower and deeper ; the spines, 

 likewise, are longer, narrower, and differently sculptured. 



The primary spines of Cidaris florigemma most nearly resemble those of Cidaris coronata, 

 but their length, and the regularity of their rows of granules, serve to distinguish them 

 from those of that species. The only other species of Cidaris with which it is necessary to 

 compare them, is that of Cidaris SmitJiii, which sometimes occurs with Cidaris florigemma 

 in the same rock in England. Although the tests of these two species resemble each other 

 when of the same size, still the spines show that they belong to two distinct groups of 

 urchins, the stems of Cidaris florigemma being thick and massive, whilst those of Cidaris 

 Smithii are long and slender, tapering very gradually from neck to point ; the surface is 

 covered with elevated longitudinal ridges, covered with sharp, forward-directed prickles ; 

 these spines are sometimes nearly twice the length of those of Cidaris florigemma. (PI. V, 

 fig. 5.) The test of Cidaris Blumenbachii has the areolas deeply excavated; the bosses 

 not very prominent, the summits all sharply crenulated, and the tubercles small; the 

 granules forming the scrobicular circle are not larger than those of the small granulation 

 filling the miliary zone. These characters are so completely diagnostic, that it is impos- 

 sible to mistake the true German form for Cidaris florigemma, when the two urchins are 

 placed side by side for comparison. If to these characters of the test, however, are added 

 those obtained from the spines, we learn how entuely distinct Cidaris Blumenbachii is 

 from Cidaris florigemma. 



Locality and Stratigraphical position. — Cidaris florigemma is found in fine preserva- 

 tion in the Coralhne Oolite of Wiltshire, Berkshire, Oxfordshire, Yorkshire, and Dorsetshire. 

 The specimens figured in PI. II were collected near Calne, in Wilts ; and I have seen 

 a beautiful specimen found in that formation at Hildenly, near Malton, Yorkshire, 



7 



