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



Salenians by having numerous nearly equal-sized tubercles upon 

 the ambulacral and interambulacral arese. The pores are differ- 

 ently arranged in the avenues in the different genera ; the apical 

 disc consists of five ovarial and five ocular plates. 



Genus Diadema, Gray. 



Test thin, of a circular or pentagonal form, more or less de- 

 pressed, supporting perforated tubercles raised on mammillary 

 eminences with slightly crenulated summits. The ambulacral 

 arese are wide, straight, and well developed ; each area has two 

 rows of primary tubercles nearly as large as those occupying the 

 interambulacral area?. The pores are set in pairs, uniformly 

 superimposed on each other, with one or two exceptions, where 

 they fall into double files. The interambulacral area? have two 

 rows of primary tubercles, and sometimes ranges of secondary 

 tubercles placed external to them. The mouth is large and 

 decagonal, with shallow marginal notches. The five ovarial plates 

 have an elongated hexagonal form ; the madreporiform is larger 

 than the pairs of plates ; the five ocular plates are small and tri- 

 angular, and are lodged at the summits of the ambulacra between 

 the re-entrant angles formed by the ovarial plates. The spines 

 are long, slender, and subulate, and of a very uniform size 

 throughout. 



Diadema depressum, Agassiz. PI. XII. fig. 2 a, b } c, d. 



Syn. Diadema depressum, Agassiz and Desor, Catalogue raisonne des 

 Echinides, Ann. des Sciences Nat. 1846; Cotteau, Etudes sur 

 Echinides Fossiles, p. 43. t. 2. 



Test pentagonal, depressed; ambulacral arese convex and promi- 

 nent ; interambulacral arese flattened ; two rows of nearly 

 equal-sized primary tubercles in both arese; secondary tu- 

 bercles absent or rudimentary ; mouth large and slightly de- 

 cagonal. 



Height T 5 oths of an inch, breadth 1 inch and ygth. 



Description. — The ambulacral arese of this Urchin are rather 

 more than one-half the breadth of the interambulacral arese, and 

 have from ten to twelve pairs of well-developed primary tubercles 

 separated by a zigzag line of small granulations. The interam- 

 bulacral arese are nearly of a uniform breadth throughout ; there 

 are about ten pairs of tubercles in each area ; in consequence of 



Desmoulins ; Echinopsis, Agass. ; Arbacia, Gray ; Eucosmus, Agass. ; Cce- 

 lopleurus, Agass.; Codiopsis, Agass.; Mespilia, Desor; Microcyphus, Agass.; 

 Salmacis, Agass. ; Temnopleurus, Agass. ; Glypticus, Agass. ; Polycyphus, 

 Agass. ; Amblypneustes, Agass. ; Boletia, Desor ; Tripneustes, Agass. ; Ho- 

 lopneustes, Agass. ; Echinus, Linn. ; Pedina, Agass. ; Heliocidaris, Des- 

 moulins. 



