FROM THE GREY CHALK. HI 



PsEUDODiADEMA Brongniarti, Agas&iz, 1840. PI. XX, fig. 2 a — c; XXI a, figs. 2 



a—f, 3, 4 ; XXI b, figs. 1—3 a—e. 



Tetbagramma Brongniakti, Agassiz. Desc. des Echinides fossiles de la Suisse, t. ii, 



p. 25, pi. xiv, figs. 4—6, 1840. 



— Agassiz et Besor. Catal. rais. des Echinides, Ann. Sc. 



Nat., 3e ser., t. vi, p. 350, 1846. 

 — — Bronn. Index Paleeontologicus, p. 1261, 1849. 



Diadema — B'Orbigny. Prodrome, t. ii, p. 142, Et. 19, No. 328, 



1850. 



— — Benevier. Mem. Geol. sur la Perte du Rhone, p. 32, 



1853. 



— — Forbes. In Morris's Catalogue of British Fossils, 2nd 



ed., p. 76, 1854. 

 Pseddodiadema — Besor. Synopsis des Echinides fossiles, p, 74, 1856. 



Diadema — Woodward. Mem. of the Geol. Surv., Decade V, 1856. 



_ — Pictet. Traite de Paleontol., 2e ed., t. iv, p. 244, 1857. 



Pseudodiadema — Bvjardin et Hupe. Hist. Nat. des Zoophytes, Echino- 



dermes, p. 498, 1862. 

 — — Cotteau. Paleontologie Francaise, Terrain Cretace', t. vii, 



p. 456., pi. 1109, 1865. 



Test large, subcircular, elevated ; sides tumid, depressed at the upper surface, narrow, 

 rounded, and contracted on the under surface ; ambulacral areas narrow, with two rows of 

 tubercles, separated by a double series of small granules of unequal sizes; poriferous zones 

 narrow, straight ; pores round, in single pairs from the peristome to the ambitus, and 

 bigeminal thence to the disc-opening ; inter-ambulacral areas wide, with four rows of 

 primary tubercles, nearly identical in size with those of the ambulacra, and two short 

 rows of very small secondaries near the zones. Mouth-opening small, in a considerable 

 depression ; peristome narrow, with feeble and nearly equal-sized entailles. 



Dimensions. — a. — Height six tenths of an inch ; transverse diameter one inch and a 

 half. B. — Transverse diameter two inches and a half. 



Description. — The test of this species exhibits so close a resemblance in many of its 

 anatomical details to that of Pseudodiadema variolare, that, were it not for some differences 

 in the size, shape, and development of the shell, and in the structure of the ambulacra, I 

 should hesitate to separate it from that form ; after all, these differences may not be 

 specific, but may have arisen from habitat and other physical conditions. A test of each 

 species, with spines attached, for the purpose of comparison, is still with me a desideratum ; 

 however, as this Urchin is considered by most authors to be distinct from P. variolare, I 

 shall describe the fossils I have figured under the name D. Brongniarti, Agas. These 



