258 Dr. T. Wright on the Cidaridse of the Oolites. 



The interambulacral arese are twice and a half the diameter of 

 the ambulacral ; in each column there are from six to seven plates, 

 the three or four inferior of which support moderate-sized mam- 

 millary eminences with crenulated summits, from the centre of 

 which a large prominent deeply perforate tubercle rises. The 

 areolae are smooth and gently inclined, and around their circum- 

 ference fifteen small granules are set. The three superior plates 

 are destitute of mammillary eminences, and in lieu thereof have 

 clusters of granules on each plate similar to those occupying the 

 ambulacral arese. There are from two to five such granules pro- 

 truding from the upper surface of the test; they are arranged in 

 pairs, or form triangular, quadrangular or pentagonal figures. 

 The apical rosette is well developed ; the ovarial plates are large 

 and marked with a depression near their centre, and their in- 

 ternal borders are slightly raised. The madreporiform plate is 

 larger than the pairs of plates, and its centre is occupied with a 

 porous structure. The ocular plates are large and heart-shaped, 

 with a depression down the centre of each plate. In the speci- 

 men before me the plates of the apical rosette are devoid of other 

 sculpture. 



The base is flat, the mouth large and decagonal, the opening 

 being more than half the diameter of the test at its equator. 



The spines are unknown. 



Affinities and differences. — This Urchin differs from H. inter- 

 media in the absence of tubercles from the upper part of the in- 

 terambulacral arese, in the form and size of the ovarial and ocular 

 plates, and in the form and structure of the granules covering 

 the ambulacral arese. It is distinguished from H. alpina by the 

 absence of the close-set granulations covering the convex ambu- 

 lacra of that Urchin. It has some resemblance to H. icaunensis, 

 but is distinguished from it by the small number of its primary 

 tubercles, and the warty figures which take the place of the 

 tubercles on the upper surface of the test. 



Locality and stratigraphical range. — From the Inferior Oolite 

 of Dundry. Imperfect specimens, probably belonging to this 

 species, have been collected from the upper beds of Leckhampton. 



Hemicidaris confluens, M'Coy. 



Syn. Hemicidaris conjluens, M'Coy, Annals of Nat. Hist. vol. ii. 

 New Series, p. 411. 



Test spheroidal, much-depressed; ambulacral arese slightly con- 

 vex and nearly straight, with two alternate marginal rows of 

 small microscopic mammillated and perforated tubercles, four 

 pairs of larger tubercles at the base ; intermediate surface 

 covered with small close-set granulations ; interambulacral 

 arese with three pairs of large tubercles at the middle, four 



