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



History. — First figured and described by Goldfuss in his f Pe- 

 trefacta Germanise/ and afterwards by Agassiz in his ( Description 

 des Echinodermes Foss. de la Suisse/ and now figured and de- 

 scribed as a British fossil from the Inferior Oolite near Chelten- 

 ham for the first time. 



Genus Hemicidaris (Agassiz). 



Test subglobose, more or less flattened at the poles. Ambu- 

 lacral areas narrow and sinuous, furnished with primary tubercles 

 on the lower fourth part of each area, which suddenly diminish 

 into small tubercles or granules above, set more or less closely 

 together like those in the areas of Cidaris. Interambulacral areas 

 much larger than the ambulacra!, widest at the equator of the 

 test and narrowest at the poles ; around the circumference of the 

 mouth they are about the same breadth as those of the ambu- 

 lacral areas. 



The primary tubercles of the interambulacral areas are raised 

 upon large prominent mammillary eminences, having a crenu- 

 lated margin encircling the base of the tubercle ; the equatorial 

 plates carry the largest mammillary eminences. Pores biserial, ex- 

 cept near the mouth, where they are triserial. Mouth large, with 

 decagonal indentations around its circumference. Anus central, 

 surrounded by a solid circle of ten plates which are often well 

 preserved. The five ovarial plates are larger and perforated at 

 their summits. The single or madreporiform plate is the largest ; 

 it has a more porous structure, and is differently sculptured from 

 the pairs of plates. The five ocular plates are small and trian- 

 gular : both ovarial and ocular plates are covered with minute 

 granulations. 



Spines of two orders : the primaries are long, cylindrical, and 

 mostly of considerable dimensions, the secondaries are small and 

 compressed. This genus differs from the true Cidaris in the 

 bases of the ambulacral areas supporting primary tubercles. He- 

 micidaris thus forms a type of structure intermediate between 

 Cidaris and Diadema. In Hemicidaris the mouth is decagonal, in 

 Cidaris it is circular. 



All the species are fossil, and characterize the middle and 

 upper stages of the oolitic rocks. Some are found in the Neo- 

 comian and in the Chalk. 



Hemicidaris intermedia, Fleming. 



Syn. Cidaris papillata, var. Park. Org. Rem. pi. 1. fig. 6. vol. iii. 



Cidaris intermedia, Fleming, Brit. Animals, p. 478. 



Hemicidaris crenularis, Morris, Cat. Brit. Foss. p. 53 ; Strickland 



and Buckman, Geol. of Chelt. 

 Hemicidaris intermedia, Forbes, Brit. Org. Rem. Decade 3. pi. 4. 



