Dr. T. Wright on the Cidaridas of the Oolites. 255 



sides, and its transverse diameter is one-half more than its height. 

 The interambulacral areas are furnished with two rows of large 

 primary tubercles ; in each range there are from six to seven 

 tubercles, which attain their greatest development at the equator 

 of the test, and diminish in size near the anal and buccal open- 

 ings. The mammillary eminences supporting the tubercles are 

 large, prominent, and surrounded by areolae. The tubercles are 

 small and perforated ; one row of granules separates the large 

 tubercles from the poriferous avenues, and a double row occupies 

 the middle of the areas. The lateral boundaries of the areolae 

 are surrounded by a semicircle of granules, whilst the upper and 

 lower boundaries of the same blend into each other. 



The ambulacral areas are narrow, slightly undulated, and fur- 

 nished through nearly all their extent with a double row of small 

 tubercles, which are not very apparent, but are larger on the 

 sides than at the apex of the areas ; between the size of these and 

 the three pairs of tubercles at the base a sensible difference exists. 

 The mouth-opening is large, and is one-half the diameter of the 

 test ; it is of a decagonal form with the margin deeply notched. 

 The apical disc is not preserved and the spines are unknown. 



Affinities and differences. — The Hemicidaris icaunensis in its 

 general form and characters closely resembles the H. intermedia ; 

 it is distinguished from the latter by having the primary tuber- 

 cles of the interambulacral areas less prominent, by the ambu- 

 lacral areas being less waved, and in having the basal tubercles 

 much smaller. This character assimilates H. icaunensis to H. 

 Thurmanni, but it is sufficiently distinguished from that Urchin 

 by its greater height, less undulated ambulacra and the greater 

 number of tubercular plates in the interambulacral areas. 



Locality and stratigraphical range. — This rare species was ob- 

 tained by Mr. Lycett from the Great Oolite of Minchinhampton. 

 M. Cotteau collected it in France from the superior beds of the 

 Bathonian stage at Chatel-Censoir, and M. Rathier found it in 

 the Forest marble of Chatel -Gerard, where it is likewise rare. 



History. — This species was first figured and described by M. 

 Cotteau*, and was provisionally identified by Prof. Forbesf; it 

 is figured in plate A. fig. 9. of the ' Monograph of Great Oolite 

 Fossils ' to be published by the Palasontographical Society. The 

 specimen that has come under our notice is so imperfect that we 

 have followed M. Cotteau's description. 



* Echinides Foss. du Departement de l'Yonne, tab. iii. p. 56. 

 f Memoirs of the Geological Survey ; Brit. Organic Remains, Decade 3. 

 Description of plate 5. 



