FROM THE CORNBRASH. 247 



rows of granules : the bosses of the four middle pairs of tubercles are moderately large 

 (fig. 3 e), they have small areolas, which are nearly divided from each other by horizontal 

 lines of granules ; in some they are absent, and the areolas are then confluent. 



The apical disc (fig. 3 a) is seldom preserved ; in one specimen however (PI. XVII, fig. 5), 

 it is seen to consist of an anterior and posterior pair of ovarial plates, and a single rudi- 

 mentary ovarial ; the sur-anal plate is composed of two unequal-sized pentagonal pieces, 

 united with the anterior and posterior ovarials, and six or seven small pieces which form an 

 arch, extending from the right to the left posterior ovarials, and completing the anterior wall 

 of the anus (PI. XVII, fig. 5) ; the posterior pair of ocular plates form the lateral, and 

 the single ovarial the posterior boundary of the vent, which is transversely oblong, slightly 

 excentral (fig. 3 a), and projects into the single inter-ambulacrum. The ocular plates are 

 heart-shaped and of moderate size, and the eye-holes are very minute ; the surface of all 

 the discal elements is covered with small close-set granules (PI. XVII, fig. 5). 



The tumid sides are gently rounded towards the base, which is concave ; the mouth 

 opening is small, being less than two fifths the diameter of the test ; the peristome 

 is unequally decagonal, the ambulacral being one third larger than the interambulacral 

 lobes. 



The fragment of a primary spine shows that it was smooth and cylindrical, and, judg- 

 ing from its thickness, must have been long ; the secondary spines are short and prickle- 

 shaped, and are sculptured with fine longitudinal lines. 



Affinities and differences. — This species so very closely resembles Biadema (Acrosalenia) 

 LamarcJcii, Desmoulins, that at one time I thought them identical, and stated as much 

 in a letter to M. Desor ; but from a more attentive study of the form, I now consider 

 them distinct. A. Wiltonii has a more concave base, and a smaller mouth opening 

 than any other form of the genus at present known. It differs from A. pustulata in 

 having a wider miliary zone, a greater number of much smaller granules, a more concave 

 base, and a smaller mouth opening ; the same group of characters serve to distinguish it 

 from A. hemicidaroides and A. Lycettii. 



Locality and Stratigraphical position. — This urchin was collected many years ago by 

 Dr. William Smith, from the Cornbrash of Wiltshire. My type specimens were found by 

 Mr. William Buy, in the Cornbrash, near Sutton-Benger, Wilts, where it appears to be 

 very rare, and is known as " the small-mouthed Cidaris." Mr. Bravender obtained it 

 from the Bradford Clay, near Cirencester. 



I dedicate this species to my esteemed friend, John Wilton, Esq., of Gloucester, with 

 whom I have spent many hpapy days exploring the Natural History of different parts of 

 Gloucestershire. 



