﻿208 



DISCOIDEA 



Desor describes and figures three fine marginal incisions visible below and in profile in 

 each column of plates, and which assume a different form in the single inter-ambulacrum 

 where they are only two in each column ; they are here wider and deeper than those in- 

 the pairs of the inter-ambulacra. 



PI. XLVII, fig. 3, is the drawing of the base of a mould of a monstrosity of this 

 species, in which there are only four ambulacra seen from below. 



Affinities and Differences. — Discoidea cylindrica cannot be mistaken for any of its 

 congeners, as it is readily identified by its great height, its inflated and sub-cylindrical 

 form, its perfectly flat base, and by the proportionate smallness of the oral opening and 

 the vent. Discoidea Favrina is the nearest allied form ; this, however, differs from 

 D. cylindrica in having a larger vent placed nearer the border, and in having the inter- 

 ambulacral plates much larger ; each plate, in vertical height, corresponding to six 

 ambulacral plates. 



Locality and StratigrapMcal Position. — This species occurs in the Upper Chalk of 

 Norwich and Holt (Mr. S. Woodward, Sen.), Chalk Marl and Lower Chalk at Hamsey, 

 near Guildford, Markham Gayton, Charing, Lewes, Dover, Burham, near Maidstone, 

 Speeton, Yorkshire ; the Chloritic Marl, near Chardstock, and in the Red Chalk, in the 

 highest of the tinted bands at Speeton Cliff, Yorkshire, at the part where the pink and 

 white sems alternate (Rev. T. Wiltshire). 



Foreign Distribution. — In France, in the 'Etage Cenomanien,' according to M. 

 Cotteau, Rouen (Seine-Inferieure) ; Pourrain, Saint-Sauveur (Yonne) ; La Fauge pres le 

 Villard-de-Lans (Isere) ; Saint- Aignan en Vercors (Drome) ; Castellanne (Basses- Alpes). 

 In Germany, in the Lower Planer (stage with Ammonites Flhotomagensis) — Chalk-marl 

 of English authors ; at Langelsheim near Brunswick (Strombeck) ; at Rethen, near Hilde- 

 sheim (Romer), near Paderborn (Goldfuss), from the Gault of the Mountain of Fis, 

 according to Desor, and from the Etage Albien, Cheville, Alpes Vaudoises = Lower 

 Chalk (Renevier). 



Discoidea minima, Agassiz, 1840. PI. XLVII, fig. 4 a — h. 



Discoidea minima, Agassiz. Catal. Syst. Ectyp. Foss., p. 7, 1840. 



— — Desor. Monogr. des Galerites, p. 56, pi. viii, figs. 1 — 4, 1842. 



— — Morris. Catalogue of British Fossils, p. 52, 1843. 



— — Agassiz and Desor. Catal. raisonne des Echinides, 1847. 



— — Morris. Catalogue of British Fossils, p. 77, 2nd ed., 1854. 

 _ — McCoy. Contributions to British Palaeontology, p. 67, 1854. 



— — Cotteau. Paleontol. Franchise, t. vii, pi. 1012, figs. 1 — 7, 1866. 



Diagnosis. — Test small, sub-circular; height and length equal; upper surface 

 inflated ; base slightly convex ; border round ; ambulacra half the width of the inter- 

 ambulacra ; tubercles form regular series on the sides of the areas ; mouth and vent, 

 moderately wide. 



