FROM THE LOWER GREENSAND. 87 



The spines rarely attain the length of the diameter of the test ; in general they are 

 much shorter, cylindrical, or needle-shaped, and have a prominent, milled ring near 

 the articulating head ; the rim of the acetabulum is crenulated, and the socket perforated ; 

 the surface of the stem is sculptured with delicate longitudinal lines. 



The Pseudodiademata are all extinct, and found in the Liassic, Oolitic, Cretaceous and 

 Tertiary rocks. 



Pseudodiadema differs from Biadema in having solid spines, with a smooth surface, 

 the sculpture, in most cases, consisting of microscopic, longitudinal lines ; whilst in 

 Diadema the spines are tubular, and have oblique annulations of scaly fringes on their 

 surface. Pseudodiadema differs from Cyphosoma^ a Cretaceous genus, in having the 

 tubercles always perforated, those of Cyphosoma being imperforate. It differs from 

 Hemipedina in having a small apical disc, and tubercles with crenulated bosses, those of 

 Hemipedina being smooth ; and from Pedina in having the pores unigeminal or 

 bigeminal, those of Pedina being arranged in triple, oblique pairs. 



Pseudodiadema may be divided into two sections, from the different manner the 

 pores are arranged in the zones. In one group the pairs of pores form a single 

 file throughout; in another the pores are more numerous, and crowded together in 

 the upper part of the zones. Professor M'Coy has proposed the genus Diplopodia for 

 the latter. It may be objected^ however, that the crowding together of a greater number 

 of pores in a zone is, at most, a sectional and not a generic character, inasmuch as the 

 arrangement is subject to great variation in the diplopodous species themselves, and is, 

 moreover, often only an adult development. 



A. — Species from the Lower Greensand. 

 Pseudodiadema rotulare, Ayassiz. PI. XIV, figs. 3 a, b, c. 



Diadema ROTULARE, Agassiz. Mem. des Sn. nat. de Neuchatel, vol. l,p. lo!), tab.xlv, 



figs. 10—12, 1836. 



— — Bes Moulins, £tudes sur les Echinides, p. 316, No. 25, 1837. 



— ORNATUM, Jgassi:. Catal. Syst. Ectyp. foss. Musei Neoc, p. 8, 1840. 



— ROTULARE, Jffossiz. Descdpt. des Echin. foss. de la Suisse, part 2, p. 4, 



tab. xvi, fig. 1—5, 1840. 



— MACROSTOMA, Agassiz. Ibid., p. 10, tab. xvi, fig. 22 — 26, 1840. 



— ROTULARE, Agassis et Desor. Catal. Raison. des Ecbinides, Ann. des Sc. nat, 



3e serie, t. vi, p. 346, 1846. 



— MACROSTOMA, Agussiz ct DesoT. Ibid., p. 347, 1846. 



— — Bronn. Index Palaeontologicus, p. 418, 1846. 



— CORONA, Gras. Oursins foss. de I'lsere, p. 33, pi. i, fig. 21 — 23, 1848. 



— ROTULARE, Marcou. Ilecherch. geol. sur le Jura Salinois, Mem. Soc. Geo), de 



France, Ire serie, t. iii, p. 143, 184S. 



