84 HEMICIDARIS. 



separated from each other (fig. 6d); but there are no granules between the boss and the 

 poriferous zone (fig. 6 d). 



The apical disc is large, and beautifully preserved in one of the two specimens 

 before me ; it forms a considerable prominence on the upper surface of the test (fig. 6 6) ; 

 the genital plates have a heptagonal form, are very thick, and have a remarkable depression 

 near their centre, at the bottom of which the oviductal tubes appear to open (fig. 6 e) ; the 

 anterior pair of plates are the largest, the posterior pair are rather smaller, and the single 

 plate is the smallest ; the right antero-lateral, carrying a small madreporiform body in its 

 depression, is the largest of the series ; the ocular plates are small heart-shaped bodies ; 

 the vent is slightly pentagonal, excentral, and posterior. 



The mouth opening is one half the diameter of the test, it has a decagonal form, and 

 the peristome is slightly notched ; the ambulacral lobes are larger than the inter-ambulacral ; 

 the base is flat, and the tubercles of both areas on this region of the test are nearly all of 

 the same size (fig. 6 c) ; the difierent appearance which the test presents when viewed 

 on its under surface (fig. 6 c), and on its upper surface (fig. 6 5), is very marked indeed. 



Affinities and differences. — The only Hemicidaris this little form can possibly be 

 mistaken for is Hemicidaris minor (PI. Ill, fig. 5), with which it is associated in the same 

 bed and locality ; but it is readily distinguished from that species by its depressed form, 

 and by the inter-ambulacral areas being crowded with large prominent tubercles, raised 

 on very prominent bosses ; the ambulacral areas are likewise much narrower above, 

 and the space for the semi-tubercles is much wider below; in fact, the remote- 

 ness of the tubercles from each other in Hemicidaris minor, is as good a diagnostic 

 character of that species, as the disproportionate largeness of the tubercles, and their 

 consequent crowding near the circumference of the test, is of Hemicidaris Hamsayii. 



Locality and Stratigrapliical position. — This rare little urchin was collected from the 

 shelly ferruginous beds of the Great Oolite, at Sham Castle, near Bath, associated with 

 Hemicidaris minor, Acrosalenia spinosa, and other Great Oolite forms. I dedicate this 

 species to my friend, Professor Andrew C. Ramsay, P.R.S., Director of the Geological 

 Survey of Great Britain. 



Hemicidaris Bravenderi, Wright, nov. sp. Pl.V, fig. 1 a, b, c, d, e,f. PI. XI, fig. 3 a, b, c. 



CiDAKis CRENULAKis. Murchisoii, Gcology of Cheltenham, 2d edit., Buckmau and Strick- 

 land, pi. 13, p. 73. 



Test sub-globular, flattened at the base ; ambulacral areas nearly straight, with two 

 marginal rows of minute, well spaced out, perforated tubercles, and six pairs of moderate 

 sized semi-tubercles ; inter-ambulacral areas with two rows of moderate sized primaiy 



