FROM THE GREY CHALK. 113 



The pores are scarcely bigeminal near the summit. The primary tubercles are 

 numerous, nearly uniform in size, and closely set together ; the mouth-opening is very 

 small, and lies in a deep depression. 



Pseudodiadema variolare has the test in general more depressed, the base wider, and 

 the pores more distinctly bigeminal in the upper fourth of the zones ; the base is wider, 

 more convex, and less contracted than in Ps. Bronpniarti. 



LocalHy and titrati(/raphical Position. — Pseudodiadema Bron(jniarti has been col- 

 lected from the Grey Chalk near Folkestone, from which stratum all the large tine speci- 

 mens in the British Museum, and those in the Rev. T. WiUshire's cabinet figured in 

 this Monograph, have been obtained. The Red Chalk of Hunstanton Cliff has yielded 

 a few examples, two of which, from Mr. Rose's and the Rev. T. Wiltshire's collections, 

 are figured in PI. XXI b. Forms referred to this species have been collected from 

 the Chalk-marl of Maiden Bradley, Dorset, and the Chloritic Marl, Somerset. 



Foreign Localities. — Professor Pictet, of Geneva, kindlv gave me several type speci- 

 mens of this Urchin collected from the Gault of the Perte du Rhone (Ain), which so much 

 resemble Professor Agassiz's figures of this species from the same locality that my speci- 

 mens might have been the originals of the drawings in his ' Bchinodermes foss. de la 

 Suisse.' It is found likewise at Escragnolle (Var), Montngne des Fis (Savoie), where it is 

 an abundant fossil in the Etage Albien or the Gault. M. Desor, in addition to these 

 localities, gives Clar and La Presta, as places where this species is abundant. 



History. — First figured by M. Al. Brongniart^ in his ' Description de la Perte du Rhone,' 

 under the name Cidariles variolaris {?), as a characteristic fossil of the Craie INIarneuse ; 

 afterwards (1S40) it was described and figured by Professor Agassiz in the ' Echinodermes 

 foss. de la Suisse' as Tetrngramma Brongniarti ; afterwards (1856) it was removed by 

 M. Desor into his genus Pseudodiadema, where it now remains. 



Genus — Pedinopsis, Cotteau, 1803. 



Test large, round, inflated, sometimes subconical. Poriferous zones wide and straight ; 

 the pores bigeminal throughout, forming at the upper surface and ambitus two distinct 

 rows, which become more blended together at the infra-marginal region, and are distinct 

 at the base. Tubercles of both areas small, and nearly the same size ; summit of the 

 boss finely crenulated, and the raammillon perforated ; the tubercles disposed in regular 

 rows, the number varying in the different species, and always diminishing as they 

 approach the summit ; coronal plates long, narrow, and granular ; mouth-opening large, 

 peristome moderately developed, circumference slightly incised, apical disc small, sub- 

 circular, elements feebly united, absent in the specimens known. 



' 'Description geologique des Environs de Paris,' troisieme edition, 1835, p. 174, pi. m, fig. 9. 



15 



