128 PSEUDODIADEMA 



* CiDARis DiADEMA. YouDg and Bird, Geological Survey of the Yorkshire 



Coast, pi. 6, fig. 3, p. 212, 1828. 



— MONILIPORA. Phillips, Geology of Yorkshire, p. 127, 1829. 

 DiADEMA PSEUDODIADEMA. Wright, Annals and Magazine of Natural History, 



2d series, vol. viii, pi. 12, fig. 1 a, b, c, p. 271, 1851. 



— — Cotteau, Etudes sur les Echinides Fossiles du departe- 



ment de I'Yonne, pi. 17, fig. 1, p. 142, 1852. 



— — Salter, Memoirs of the Geological Survey, Decade V, 



pi. 2. 

 PsETJDODlADEMA HEMisPHiERicuM. Desor, Synopsis des Echinides Fossiles, t. 13, fig. 4, 



p. 68, 1854. 

 DiADEMA PSEUDODIADEMA. Morris, British Fossils, 2d ed., p. 77, 1854. 



Test hemispherical, depressed on the upper surface, flat below ; ambulacral areas with 

 two marginal rows of primary tubercles, eighteen to twenty in each row ; poriferous zones 

 narrow, straight, with a moniliform line between the pores ; inter-ambulacral areas with 

 two rows of primary tubercles, fifteen in each row, and four rows of secondary tubercles, 

 one row flanking each side of the primary rows ; apical disc small, anal opening obliquely 

 oblong. Mouth opening large, peristome decagonal, unequally lobed, and deeply notched ; 

 spines long and needle-shaped, finely sculptured with longitudinal lines. 



Dimensions. — Height, one inch and one quarter of an inch ; transverse diameter, two 

 inches and four tenths of an inch. 



Description. — Lamarck's description of his original Cidarites jpseudodiadema was so 

 meagre and ill defined, that great doubts were entertained by Desmoulins as to the urchin 

 meant by the illustrious author of the ' Histoire Naturelle des Animaux sans Vertebres ;' 

 and, unfortunately, M. Dujardin has added nothing in the new edition of that great work 

 to the original text of this species. It would have been impossible for me to have 

 identified it, had not my excellent friend M. Michelin sent me a small type specimen 

 of Lamarck's species from the CoralKne Oolite of Commercy Meuse, which is identical 

 with our English forms of the same size. 



Pseudodiadema hemispharicum may justly be regarded as the type of the genus, exhibit- 

 ing, as it does, the characters of the group in a most satisfactory manner. It appears, 

 moreover, to have had a wide geographical range, as it is found in the same stage of the 

 CoraUine Oolites in Switzerland, in difierent departments of France, and in England. 



When M. Agassiz pubhshed his ' Prodromus,' he was unacquainted with the original 

 of Lamarck's species, a subsequent comparison of specimens, however, convinced 

 him that the urchin he had collected in the Swiss Jura, and named Diadema hemisphari- 

 cum, was identical with Diadema pseudodiadema, Lamarck, and that his Diadema trans- 

 versmn was merely a distorted form of the same urchin.* It is probable that Diadema 



* 'Description des Echinodermes Fossiles de la Suisse,' partie 2% p. 12. 



