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



lacral arese are slightly convex, and taper very uniformly from 

 the base to the summit. The lower half of the arese is occupied 

 by eight rows of primary tubercles, four in each column; at the 

 base and for a short distance up the sides of the test, these 

 tubercles are of a uniform size, but beyond this two rows only 

 maintain their development, and numbering twenty-four pairs of 

 primary tubercles in each area ; the two external rows and the 

 single internal row of tubercles are arrested in their develop- 

 ment, and therefore become of a secondary size. The tubercles 

 of both area? are surrounded by a smooth areola encircled by a 

 groove, on the external margin of which a wreath of small granules 

 is disposed, reminding us of the granular zone surrounding the 

 primary tubercles in the genus Cidaris. The inter tubercular 

 spaces of the arese are filled up with small granules. The inter- 

 ambulacral arese are separated along the median line by a some- 

 what depressed furrow, which is most conspicuous between the 

 equator and the anal pole. This furrow arises from the con- 

 vexity of the new-formed plates, and becomes less evident when 

 the plates attain a greater width; this depression is likewise 

 destitute of granulations, and affords a good specific character 

 for this Urchin. The poriferous avenues are of uniform width 

 on the sides of the test ; they become slightly contracted at the 

 basal angle, and expand from that point to the margin of the 

 mouth-opening. The avenues have three pairs of holes disposed 

 obliquely throughout, but increased to four or five pairs to fill 

 up the increased spaces of the avenues in the vicinity of the 

 mouth. The ovarial and ocular plates are in general preserved. 

 The anal opening is always eccentrical, which gives the summit 

 of the test an irregular form ; the opening is placed forward, so 

 that the madreporiform plate occupies nearly the centre of the 

 anal polar axis. The pairs of ovarial and ocular plates are small 

 and imperfectly developed. 



The base is concave, and in this region all the primary tubercles 

 of the interambulacral arese attain their full development. The 

 mouth-opening is large and decagonal, occupying nearly one- 

 half the diameter of the base; the circumference is deeply 

 notched with ten indentations which extend into the interambu- 

 lacral arese, and have their borders reflexed. 



The spines are small, delicate, and subulate, but are very 

 seldom found in connection with the test. 



Affinities and differences. — We have, through the courtesy of 

 Mr. S. P. Woodward, compared our Urchins with the typical 

 specimens of E. perlatus in the Brit. Mus., and through the 

 kindness of Professor Forbes with a specimen of E. germinans, 

 sent by Mr. Phillips from Yorkshire ; from this examination it 

 is certain that the Gloucestershire and Yorkshire Echini are 



