FROM THE LOWER GREEXSAND. 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 Paeudodiademata are all extinct, and found in the Liassic, Oolitic, Cretaceous and 

 Tertiary rocks. 



Pfseudodiadema differs from Diadema in having solid spines, with a smooth surface, 

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

 Diadeitia 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 Cyphoxoma being imperforate. It differs from 

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

 Hemijjpdtna 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 Biplopodia 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 front the Lower Greemand. 

 PsErD0Di.\DE3iA EOTCLABE, Ago^fsiz. PI. XIV, figs. 3 a, b, c. 



DiADEMA EOTCLAKE, Ag(u«iz. M^m. des Sc. nat. de Xeuchatel, vol. l,p. 139, tab. xiv, 



fig?. 10—12, 183C. 



— — Bet Moulin*, £tiideg 8ar les ficliinidei", p. 316, No. 2.^, 1837. 



— OBXATCM, Jga»n:. Catal. SyBt. Ectyp. fogs. Masei Neoc, p. 8, 1840. 



— EOTCLAKE, JffOMiz. Dcfcript. des lochia, fois. de la Saitse, part 2, p. 4, 



Ub.xvi, fig. 1—5, 1840. 



— MACKOSTOMA, Ayatnz. Ibid., p. 10, tab. xvi, fig. 22 — 26, 1840. 



— KOTCLABE, Ag(u*iz et Detor. Catal. Itaison. des Echinides, Ann. dc« Sc. nat, 



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



— MACKOSTOMA, Jgassiz et Beior. Ibid., p. 347, 1846. 



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



— COKOXA, Grot. OursinB fosa. de rietre, p. 33, pi. i, fig. 21— 2.3, 1848. 



— EOTCL.vKE, Marcou. Recherch. geol. ear le Jura Salinois, M6m. Soc. Geo), de 



France, Ire fserie, t. iii, p. 14.3, 1848. 



