Dr. T. Wright on the Cidaridae of the Oolites. 177 



hind, excavated out of the single ovarial plate, which is rudi- 

 mentary, the anterior and posterior pairs of ovarial plates well 

 developed. Basal angle obtuse, from the tumidity of the sides ; 

 mouth-opening small, decagonal, with nearly equal-sized mar- 

 ginal lobes. 



Height ^ths of an inch, transverse diameter ^4ths. 



Description. - This Urchin is almost always orbicular in the 

 circumference, but the dorsal surface is more or less elevated in 

 different individuals; in general it is hemispherical and de- 

 pressed at the anal pole ; the sides are tumid and gently 

 rounded towards the mouth. The ambulacral areae preserve a 

 very uniform width through nearly all their extent. Two rows 

 of about sixteen small perforated tubercles are arranged in 

 alternate order on the margins of the areae, the eight inferior 

 pairs being larger, and the eight superior pairs gradually de- 

 creasing in size until they become quite microscopic near the 

 apex ; between the marginal tubercles a double row of granules, 

 having a sinuous disposition, occupies the centre of the area?. 

 The poriferous avenues are gently undulated ; on the sides the 

 pores are placed in pairs, but at the base, from their increased 

 numbers, they fall into triple oblique pairs. The interambu- 

 lacral area? are three times the width of the ambulacral, and fur- 

 nished with two ranges of primary tubercles from 9-10 in a 

 range ; the three basal pair are small, gradually increasing how- 

 ever in size from the mouth-margin, where they are very small, 

 to blend in with the three middle pairs, which attain their full 

 development. The seventh pair are smaller than the sixth, and 

 the eighth and ninth pairs become suddenly small and even ru- 

 dimentary. The space between the tubercles is of moderate 

 width, and is occupied by a zigzag band formed of four rows of 

 granules, those which are arranged on the borders of the areolae 

 are larger and are likewise perforated ; the primary tubercular 

 ranges are separated from the poriferous avenues by a single row 

 of small perforated granules, which form a succession of arches 

 over the ambulacral border of the areolae. The upper surface of 

 the test is covered with small close-set granulations, among 

 which the rudimentary tubercles stand conspicuous. The apical 

 disc is rather prominent and convex, and is fortunately preserved 

 in an otherwise imperfect specimen (PI. VII. fig. 4 d) : but for 

 this circumstance, we should have been in the dark touching the 

 natural relations of the species ; it is formed of an anterior and 

 a posterior pair of moderately- sized ovarial plates, and a rudi- 

 mentary single ovarial plate. The sur-anal plate is very curious, 

 and is composed of two unequal-sized pentagonal pieces united 



