278 Dr. T. Wright on the CidaridaB of the Oolites. 



the prominence of the ambulacral arese, which are not quite one- 

 half the width of the interambulacral, and have two rows of 

 tubercles throughout, and an additional row of from six to eight 

 arranged between the marginal rows at the widest part of the 

 areas near the basal angle ; the interambulacral arese are wide 

 and covered with tubercles ; at the basal angle and lower third 

 of the arese we observe eight rows of tubercles, but at the upper 

 part of the sides and near the apex there are only six rows : the 

 specimen before us being much defaced about the apices of the 

 arese, this part of the test cannot be accurately described. The 

 poriferous avenues are occupied with close-set pairs of pores ar- 

 ranged in triple oblique rows ; the basal angle is obtuse, and the 

 base is flat ; the mouth is large and indented at the circumfe- 

 rence; the ovarial and ocular plates are small, and the anus is 

 central. 



Affinities and differences. — The depressed test, pentagonal form, 

 central anus and granular surface serve to distinguish this species 

 from E. perlatus, which it much resembles. The same characters 

 form a clear diagnosis between it and E. serialis, the number and 

 smallness of the tubercles giving the upper surface of the test a 

 rugous or granular appearance. 



Locality and stratigraphical range. — This Urchin was obtained 

 from the upper ragstone of Leekhampton Hill (Inferior Oolite), 

 where it is rare ; we have only seen three specimens of the species. 



Genus Arbacia, Gray. 



Small Urchins of a subspherical form, having the test covered 

 with numerous small smooth-based imperforate tubercles, forming 

 numerous rows on the ambulacral and interambulacral arese ; the 

 pores arranged in rather deep avenues in single pairs ; base con- 

 cave; mouth large, margin with ten inconsiderable notches; 

 apical disc narrow, prominent, and ring-shaped. 



Arbacia Forbesii, Wright, n. s. PI. XIII. fig. 4«, b, c. 



Test hemispherical ; ambulacral arese narrow, with four rows of 



small tubercles ; interambulacral arese wide, divided by deep 



median depressed lines, and covered with from twenty to thirty 



rows of small nearly equal-sized tubercles. 



Height 2%ths of an inch, transverse diameter ^ths of an inch. 



Description. — The test of this beautiful little Urchin is divided 



into fifteen unequal lobes; five of these are narrow and form the 



ambulacra], and ten are wider, forming the divided interambulacral 



arese, which present an unusual appearance, having a median 



furrow descending down the centre of the arese and dividing them 



into two equal convex conical lobes ; the surface of the arese is 



