102 PSEUDODIADEMA 



E. Benett, of Norton House,* Wilts. It was first accurately described by the late Dr. 

 Woodward in his Notes on Fossil Diadems. 



The test is circular, depressed on the upper surface, concave below, and inflated at the 

 sides. The ambulacral areas are proportionally wide, with two rows of tubercles separated 

 by a double row of granules ; there are from 12 — 15 in each row, according to age ; they in- 

 crease gradually in size, from the peristome and disc towards the ambitus, where they are 

 largest; the poriferous zones are narrow and slightly undulated; the pores are in oblique single" 

 file throughout, three pairs of holes being opposite each ambulacral plate (fig. 2 a), where 

 they are magnified four diameters ; the pores at the circumference have a small tubercle 

 between each pair ; the inter-ambulacral areas are about one fourth part wider than the 

 ambulacral; they have two rows of tubercles, from 12 — 14 in each, separated by a wide 

 miliary zone, which is unequally granulated, and becomes smooth on the upper surface ; 

 a similar sparsely granulated space separates the tubercles from the poriferous zones ; 

 and at the base of the area a short row of small secondaiy tubercles extends from 

 the peristome to the angle, between the large tubercles and the poriferous zones. The 

 tubercles of both areas are nearly of the same size, those of the inter-ambulacral are 

 the largest; they have all distinct oval areolas, which are sometimes radiated, and 

 encircled by rows of very small granules. The base is inflated at the circumference, 

 and concave in the middle ; the peristome is small and deeply sunk, about one third the 

 diameter of the test ; its margin is divided by feeble entailles. The disc is absent in all 

 our specimens ; the aperture is wide and pentagonal, the angles pointing towards the 

 median suture of the inter-ambulacral areas. 



JMnilies and Differences. — This species so much resembles Pseudodiadema Michelini, 

 A"-., that it has been considered by some authors to be the same. M. Cotteau says, " P. 

 Michelini, such as we understand it, cannot be distinguished from P. Benettice, Eorb., and 

 we do not hesitate to unite the two species, which present in their form, in the disposition 

 of their tubercles, in the structure of their peristome, in a word, in the ensemble of their 

 characters, an identity almost complete." Admitting, as I do, the great similarity of the 

 tests of the two forms, still I think, by a careful study of both, we can detect characters 

 which may justify us in retaining our lamented colleague's species. P. Benett ia has the 

 tubercles smaller, the sides more inflated, the circumference of the base more convex, and 

 the mouth-opening smaller and lodged in a deeper depression. P. Michelini is a flatter 

 Urchin and the size and greater prominence of the tubercles impart to it a more spinous 

 character ; if these points of diS"ercnce in the test are borne out by any corresponding 

 difi"erence in the spines, the species may be sustained ; if not, P. Bennetice may turn out to 

 be a mere variety of P. Michelini. 



Locality and Stratigraphical Position. — This is a very abundant species in the 

 junction-beds of the Upper Greensand at Warminster, associated with P. Michelini, 



' Authoress of ' A Catalogue of WiltsLirc Fossils,' 1831. 



