Dr. T. Wright on the Cidaridse of the Oolites. 273 



genus is extinct, and the species are found in the Oolitic and 

 Cretaceous rocks. 



Pedina rotata, Agassiz. 



Syn. Pedina rotata, Agassiz, Echin. Foss. de la Suisse, pi. 15. 

 fig. 4-6. p. 36. 



Test hemispherical, depressed; ambulacral arese with two mar- 

 ginal rows of small tubercles ; interambulacral arese with two 

 ranges of primary tubercles and a few secondary tubercles ; 

 mouth small; margin slightly notched and divided into ten 

 nearly equal-sized lobes. 



Height y^ths of an inch, transverse diameter 1 inch and y^ths. 



Description. — The test of this Urchin is circular; in some 

 specimens a fullness of the ambulacral arese gives it a slightly- 

 subpentagonal outline, and it is depressed at both poles. The 

 ambulacral arese have two rows of small tubercles disposed on 

 the external border of the arese, between which small granules 

 are arranged with less regularity. The interambulacral arese are 

 twice and a half the width of the ambulacral, and furnished 

 with a double range of primary tubercles extending from the 

 mouth to the ovarial plates ; two ranges of secondary tubercles, 

 not very regular however in their arrangement, extend from the 

 mouth to near the middle of the arese. The tubercles of both 

 classes are very small in size, but perforated and crenulated on 

 the surface of the test a number of small microscopic granules 

 cluster together, and form circles around the areolae of the small 

 mammillated eminences. The poriferous avenues are narrow, 

 in which the holes are closely set in triple oblique pairs ; in the 

 three specimens before me the apical disc is either absent or con- 

 cealed by the oolitic matrix. The mouth is small and decagonal. 

 The margin is slightly notched, and divided into ten nearly equal- 

 sized lobes ; no reflection of the test is observed at the angles of 

 the notches. The spines are unknown. 



Affinities and differences. — This species is distinguished from 

 P. sublavis by the rudimentary development of the secondary 

 tubercles in the interambulacral arese, which can only be said to 

 exist at the internal side of the primaries, between the mouth 

 and the equator ; in the rest of the arese they degenerate into 

 granules. The other characters of the Urchin agree so well with 

 Agassiz' s very incomplete description, that we have not hesitated 

 to identify it with the Swiss species. Our specimens are all much 

 worn, and we know nothing of the apical disc. 



Locality and stratigraphical range. — This Urchin was collected 

 from the upper beds of the Inferior Oolite at Shurdington Hill, 

 along with Discoidea depressa and Clypeus sinuatus. 



History. — First described and figured by Agassiz in his ' Echi- 

 Ann.fy Mag. N. Hist. Ser. 2. Vol. viii. 18 



