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



Saint Honorine Ranville, where it is abundant. It has been ob- 

 tained by M. Cotteau from the ferruginous oolite, from Tour- 

 du-Pre, near Avallon, Departement de l'Yonne, which bed lies 

 upon the Calcaire h entroques, the true equivalent of the Dun- 

 dry, Cotteswold and Dorsetshire beds of the Inferior Oolite. 



History. — The D. depression was first mentioned in the ' Cata- 

 logue raisonne des Echinides * by Agassiz and Desor, but was 

 neither figured nor described by them. This however has been 

 done by M. Cotteau in his c Etudes sur les Echinides Fossiles/ 

 and is now figured and described from the English Oolites for 

 the first time. In both countries it appears to characterize beds 

 belonging to the same geological horizon. 



Diadema subangulare, Agass. 



Syn. Cidarites subangularis, Goldfuss, Petref. t. 40. f. 3 ; Rcemer, 



Verstein. t. 1. fig. 20. 

 Diadema subangulare, Agassiz, Echin. Foss. t. 17. fig. 21-25. p. 19. 



Test subpentagonal, depressed ; interambulacral arese with pri- 

 mary and secondary tubercles ; upper part of the poriferous 

 avenues with a double series of pores. 



Height ^ths of an inch, transverse diameter 1 inch and T 2 n ths. 



Description. — The test of this Urchin has a depressed and pen- 

 tagonal form arising from the prominence and development of 

 the ambulacral arese, which are narrow and contracted above and 

 furnished with ten pairs of primary tubercles. The interambu- 

 lacral area? are nearly twice as wide as the ambulacral, and are 

 adorned with two rows of primary tubercles from ten to eleven 

 in each row, and two rows of secondary tubercles arranged on 

 the sides of the primaries, but irregular both as regards their 

 number and size. Secondary tubercles are absent in the ambu- 

 lacral area?. The tubercles of both arese are proportionally large 

 and raised upon inconsiderable mammillary emineuces with de- 

 licately crenulated summits; the mammae are surrounded by ellip- 

 tical areolae, and round two-thirds of their circumference small 

 granules are disposed in circles ; although the tubercles are large 

 and spherical, the perforations are small and of inconsiderable 

 depth. Down the centres of both arese numerous small granula- 

 tions occupy the intertubercular surface of the plates, and similar 

 granular bands descend down the external margins of the interam- 

 bulacral areae ; but the distinctive character of this Urchin resides 

 in the structure of the poriferous avenues, which, instead of 

 forming, as in other Diademata, a single row of pores from the 

 base to the apex, from the equator to the apical disc they form 

 double rows of pores disposed in oblique lines. 



The mouth is large and decagonal, but the marginal notches 



