82 Dr. T. Wright on the Cidaridte of the Oolites. 



marginal tubercles, and the intermediate surface covered with 

 close-set granulations; interambulacral arese with two ranges 

 of primary tubercles from 10-12 in a range, the four central 

 pairs being alone fully developed; apical disc large, sur-anal 

 plate central, anus behind encroaching on the single ovarial 

 plate, which is rudimentary and projects far into the single 

 area ; base very concave ; mouth large, in a deep depression ; 

 primary spines long, smooth, slender and tapering ; secondary 

 spines small, hair-like, numerous. 



Height y%ths of an inch, transverse diameter y%ths of an inch. 



Description. — This elegant little Urchin is remarkable among 

 its congeners for the pentagonal outline of its test, arising from 

 the flatness of the interambulacral and the prominence and con- 

 vexity of the ambulacral arese, and for exhibiting the bilateral 

 symmetry of the Cidaridse in a very interesting manner. The 

 ambulacral arese are about one-third the width of the interam- 

 bulacral, the single ambulacrum is quite straight, and the ante- 

 rior and posterior pairs are slightly sinuous ; the apices of the 

 anterior pair curve gently backwards, and those of the posterior 

 pair upwards and inwards ; two rows of small perforated tubercles 

 alternately occupy the margins of the area, each row containing 

 from 20-24 tubercles, which gradually diminish in size from the 

 basal angle to the apex, the central and intermediate spaces 

 being covered with small close-set granules ; the pores are dis- 

 posed in single pairs throughout the avenues. 



The interambulacral arese are three times as wide as the ambu- 

 lacral, and are so much flattened that they form nearly straight 

 lines at the circumference ; the interambulacra are occupied by 

 two ranges of primary tubercles, about eleven tubercles in each 

 row, which are unequally developed in different regions of the 

 area ; the four ventral pairs are small and nearly of the same size ; 

 the four central pairs are fully developed, though not all of the 

 same volume, whilst the three dorsal pairs are quite rudimentary ; 

 the areolae of the central tubercles are transversely oblong and ver- 

 tically confluent ; a zigzag granular band, of four granules deep, 

 occupies the centre of the area, separating the two ranges of 

 tubercles from each other, and little granular bands separate the 

 tubercles from the poriferous avenues ; at the basal angle several 

 secondary tubercles are interspersed among the granulations 

 where the tubercles become rudimentary ; at the dorsal surface 

 the test is covered with small close-set granulations. The apical 

 disc is large and oblong ; it is formed of two anterior and two 

 posterior pair of well-developed ovarial plates, and a single rudi- 

 mentary ovarial, which extends far down the single interambu- 

 lacral area, and is much encroached upon by the anal opening, 

 which is in fact formed at the expense of the single ovarial plate. 



