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



Test subglobose or subconical ; ambulacral area? narrow and 

 slightly undulated, with a double row of small perforated tu- 

 bercles on the margins, and ten larger tubercles at the basis 

 of the arese ; interambulacral area; occupied with six or seven 

 pairs of primary tubercles which are raised on large closely- 

 approximated prominent mammae, with deeply crenulated sum- 

 mits ; mouth large and decagonal, margins deeply notched ; 

 spines long, cylindrical, and striated longitudinally, with a 

 tumid base ; apical rosette not prominent. 



Great Oolite specimens : height y^ths of an inch, transverse 

 diameter 1 inch and T ^ths. Coral Rag specimens : height 1 inch 

 and y^jths, transverse diameter 1 inch and T fi n ths. 



Description. — The test of this Urchin has sometimes a subglo- 

 bose form ; in other varieties the height exceeds the breadth, and 

 it then presents a subconical outline. The summit is slightly 

 depressed and the base is flat. The ambulacral arcse are narrow 

 and gently undulated ; at the base or lower third we observe five 

 pairs of moderate-sized tubercles ; at the upper two-thirds the 

 tubercles become very small and are ranged on the margins of 

 the areee ; both the large and small tubercles are mammillatcd 

 and perforated. The pores are arranged in simple pairs, but at 

 the enlarged space around the mouth additional pairs are intro- 

 duced. The interambulacral areee are nearly four times the width 

 of the ambulacral, and furnished with six or seven pairs of large 

 primary deeply perforated tubercles. The mammillary eminences 

 on which these tubercles are placed are largely developed and form 

 prominent projecting cones, the bases of which touch those of the 

 adjoining cones in the same range ; an undulating line of small 

 perforated granules separates the external border of the mammil- 

 lary bases from the poriferous avenues, and a double row of 

 similar granules forms a zigzag division down the centre of the 

 arese. The upper and lower boundaries of the areolae of the 

 mammae are confluent, whilst their outer and inner boundaries 

 are surrounded with the granules already described. 



The apical rosette is moderate in size, being about one-fourth 

 the diameter of the test ; the madreporiform plate is larger than 

 the pairs of ovarial plates ; the ocular plates are heart-shaped, 

 and the surface of the elements of this disc is studded with small 

 granules. 



The mouth is large, being half the diameter of the test ; it has 

 a decagonal form ; and the margin is deeply notched. 



The spines are of two kinds : the primary ones are long, cylin- 

 drical and tapering, and grow to double the length of the dia- 

 meter of the test, some of them measuring 3| inches in length ; 

 they, are delicately grooved in the longitudinal direction, and the 

 base is provided with a raised crenulated band, situated between 



