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DISCOIDEA. 



Tubercles very small, perforated, crenulated, and surrounded by areolae, unequal and 

 microscopic on the sides and upper surface; larger at the ambitus and base, and disposed 

 there in regular concentric rows. 



Mouth-opening small, circular, central ; peristome decagonal, marked by slight notches. 



Vent inferior, oval, sub-acuminate at both extremities, situated between the peristome 

 and posterior border; opening covered with irregular granular plates, which are very 

 small around the anal opening ; this aperture is placed near the internal angle. 



The apical disc solid, well soldered to the areal plates, and forming a slight projection 

 above the test, having in some five perforated ovarial plates, in others four perforated 

 ovarials, and an imperforate, complementary, single ovarial ; madreporiform body resting 

 on the larger antero-lateral, and extending into the centre of the disc ; ocular plates smaH 

 and well wedged into the angles of the ovarials. 



From the inner surface of the inter-ambulacral plates near the poriferous zones thick 

 shelly processes project inwards, and form internal septa, which occasion the ten charac- 

 teristic impressions near the ambitus seen on the moulds of this genus. 



Spines short, stout. 



The Discoidea very much resemble Holecfj/pus. Prof. Desor properly separated 

 the latter from the former in consequence of the absence of all internal ribs from the shell 

 of Holectypus, which likewise has a larger mouth-opening, the peristome deeper notched, 

 and the vent often marginal. 



A. — Species from the Upper Greensand. 

 Discoidea subuculus, Klein, -1734. Plate XLV, figs. 4, 5, G. 



DlSCOIDES SUBUCULUA, 

 ECHINITES SUBUCULUS, 



Discoidea subuculus, 

 echinites, 



Galeeites rotularis, 



Galerites subuculus, 



echinoneus rotularis, 

 Discoidea subuculus, 

 — rotularis, 



Klein. Nat. dispositio Echinodermatum, p. 26, pi. xiv, 

 I, m, 1734. 



LesTce, apud Klein, p. 171, pi. xiv, I, m, n, o, 1778. 

 Gmelin. Systema Natura, p. 3183, 1789. 

 Encyclop. method., Moll, et Zoophyt. Atlas, pi. 158, figs. 



14, 15, 1791. 

 Parkinson. Organic Eemains, vol. iii, p. 21, 1811. 

 Smith. Strata Ident. by Organ. Foss., pi. vii, fig. 12, 

 1816. 



Lamarck. Animaux sans Vert., t. iii, p. 21, 1811. 

 Beslong champs. Zoopli., Encyl. method., t. ii, p. 433, 

 1824. 



Defrance. Galerites, Die. Sc. Nat., t. xviii, p. 86, 1825. 

 Goldfuss. Petref. Germaniae, t. i, p. 129, pi. xlix, fig. 2, 

 1826. 



Blainville. Zooph., Die. Sc. Nat., t. lx, p. 194, 1S30. 

 Bronn. Lethsea Geogn., p. 615, pi. xxix, fig. 19, 1835. 

 Agassi:. Prodrome d'une Monogr., p. 186, 1836. 



