FROM THE CORALLINE OOLITE. 95 



The apical disc is about one fourth the diameter of the test (fig. 1 a) ; the antero-lateral 

 pairs of genital plates are the largest, the postero-lateral are smaller, and the single plate is 

 the smallest (fig. 1 g) ; the madreporiform body occupies all the surface of the right antero- 

 lateral plate, which is the largest ; the genital holes are situated near the apices of the 

 plates ; the ocular plates are heart-shaped. They form very inconspicuous elements of the 

 disc in this species. The eyeholes are very minute, and marginal, and situated opposite 

 the truncated apices of the ambulacra; (fig. 1 a) the surface of both the genital and ocular 

 plates is covered with numerous minute miliary granules, very irregularly arranged. 



The mouth opening is wide, occupying three fifths of the under surface (fig. 1 b) ; the 

 peristome is deeply notched, and the edge thereof is reflexed at the junction between each 

 inter-ambulacral series and the wide part of the poriferous zones ; the lobes of the 

 peristome are of unequal size, those of the ambulacral division being nearly one third 

 wider (fig. 1 b) than those of the inter-ambulacral. 



The five jaws are large and powerful, and are preserved in situ in several specimens 

 in my cabinet (fig. 1 b) ; each jaw has a broad, external, convex surface (fig. 1/), and 

 two lateral ridges, with intervening furrows. The jaw consists of two halves, and the 

 symphysis extends through the middle of the central ridge. 



In many specimens of this urchin the spines are admirably preserved in connection 

 with the test (fig. 1 h). The primary spines are long, tapering, and nearly cylindrical. 

 They grow in length to nearly twice the diameter of the test to which they belong, some 

 of them measuring three inches and three quarters (fig. 1 n) ; they are closely and minutely 

 striated in the longitudinal direction, but the striae are rather broader than the raised 

 interstices. The base of each spine has a narrow elevation, or second smooth ring, just 

 above the milled ring, which is prominent, and deeply crenulated (fig. 1 o) ; the acetabulum 

 is likewise crenulated round its rim. The secondary spines, which are attached to the minute 

 marginal tubercles of the ambulacra, and to the tubercles of the same size in the inter- 

 ambulacra (fig. 1 h, i), are small, compressed, and spatulate-shaped ; their surface is like- 

 wise covered with longitudinal lines (fig. 1 k), the neck is encircled with a small milled 

 ring, and a second smooth ring. Some of them measure one fifth of an inch in length. 

 On some well-preserved specimens from the Clay Beds, I have occasionally observed on the 

 surface of the test, when cleaning these specimens, small bodies resembling pedicellarias. 



Affinities and differences. — The tests of Hemicidaris intermedia and Hemicidaris 

 crenularis resemble each other so much, that it is difficult to point out the distinction. In a 

 fine specimen of Hemicidaris crenularis, from Lure Doubs, kindly sent me by M. Michelin, 

 the base is less tumid, the minute marginal tubercles on the ambulacral areas are fewer in 

 number and smaller, the primary tubercles of the inter-ambulacra are larger, and their 

 areolas are wider ; the mouth opening is likewise larger, and the shell is in general higher 

 than in Hemicidaris intermedia. But, as I have already observed, it is in the form and 

 structure of the spines that the true specific distinction is found. 



