86 Dr. T. Wright on the CidaridtE of the Oolites. 



tubercles, and two lateral incomplete ranges of tubercles ; at 

 the circumference of the test there are twelve rows of tuber- 

 cles ; apical disc well developed ; anal opening central ; base 

 flat ; mouth-opening large, almost pentagonal from the length 

 of the arches over the ambulacra ; spines unknown. 



Height |§ths of an inch, transverse diameter 1 inch and /oths. 



Description. — The distinguishing characters of this beautiful 

 Urchin are so prominent, that it forms a well-marked species of 

 a genus, in which in general specific distinctions are far from 

 being clearly defined. The test is hemispherical and elevated at 

 the vertex, and is very regularly formed ; its surface is divided 

 into fifteen nearly equal-sized lobes grouped into five divisions of 

 three lobes each, of which the ambulacrum forms the centre lobe, 

 and the half of the adjoining interambulacra the lateral lobes. 

 The distinctive character of the test consists in the median con- 

 cave depression in the centre of the interambulacra which ex- 

 tends from near the circumference to the apex, and is entirely 

 destitute of tubercles and granules ; near the circumference 

 however small tubercles occupy the space, and at this point we 

 observe twelve tubercles in a row in the interambulacra. Each 

 area is thus divided into two convex lobes ; in each lobe one com- 

 plete range of tubercles extends from the mouth to the vertex, 

 and two incomplete rows occupy each side thereof ; at the widest 

 part of the area only a few additional tubercles are introduced. 

 The ambulacra are about one half the width of the interambu- 

 lacra, and are furnished with two complete rows of marginal 

 tubercles, and two incomplete rows which occupy the central 

 parts of the sides; around the circumference of the tubercles, 

 forming the complete ranges, a series of small granules are dis- 

 posed in circles, and similar moniliform granular rings sur- 

 round the larger tubercles of the incomplete rows. The tuber- 

 cles of both areas are large and prominent, and their surfaces are 

 highly polished ; those of the base are larger than those of the 

 sides ; and, as a general remark, it may be stated that the test is 

 uniformly very granular. 



The pores are disposed in rather wide avenues in triple ob- 

 lique pairs, among which, some of the small granules en- 

 circling the marginal tubercles of the ambulacra are scattered. 

 The apical disc is large and central ; the anterior pair of ovarial 

 plates are the smallest, the posterior pair are larger, and the 

 single madreporiform plate is the largest of the disc ; the ocular 

 plates are small and pentagonal, and stand distinctly out from 

 the angles of the ovarials ; the eyeholes are small and central ; 

 the anal opening is large and transversely oblong ; the surface 

 of the ovarial and ocular plates is covered with small granules. 



