﻿204 



DISCOIDEA 



of the test in exhibiting ten notches around the margin, extending to the mouth on the 

 ventral surface, and disappearing at the lower third of the dorsal. Five of these uuite 

 near the mouth, and five continue singly to it. The centre of the prominent interspaces 

 of the latter is marked by a shallow depression. This groove corresponds to the line of 

 suture of the ambulacral plates, and its prominent sides to the pores. The ten deep 

 notches are caused by as many internal ribs, which spring from the inner sides of the 

 mouth, and run up the wall under the carinated portion of each series of inter-ambulacral 

 plates." — Forbes. 



Affinities and Differences. — After a careful comparison of D. subaculus with the four 

 following allied species, the late Professor Forbes remarks ■} — The first, Discoidea minima, 

 Agass., founded on a single example from the Chalk-marl of France, appears to be only 

 one of the less conical forms of the young of the species, such as not uncommonly occur 

 at Warminster. The second, D. pisum, Merian, is said to be exactly like D. minima, and 

 only distinguishable from it and D. subucuhs by having distinctly perforate primary 

 tubercles ; this, however, is a generic and not a specific character. The third, D. turrita, 

 Desor, is proportionally higher than D. subucuhs. The fourth, D. in/era, Desor, has 

 primary tubercles only at the base. I have little doubt, the Professor observes, that the 

 several so-called species just enumerated are only slight varieties, if as much, of D. sub- 

 ucuhs. M. Cotteau admits that D. pisum may be the young of D. subucuhs, but that the 

 three others indicated by Professor Forbes are certainly distinct. D. subucuhs differs 

 from D. Dixoni, Forb., in having smaller mouth- and vent-openings, so that the appearance 

 of the base is materially different. 



Locality and Stratigraphical Position. — This Urchin is abundant in the Upper Green- 

 sand of Warminster, and Chute Farm, Wilts ; in the junction beds of Greensand and 

 Chalk-marl at Maiden Bradley, Wilts ; the Lower Chalk at Weymouth, and the Grey 

 Chalk near Folkestone. This species is very abundant in the different type localities of 

 the Etage Cenomanien in France, see p. 189. 



b. — Species from the Lower Chalk. 

 Discoidea cylindrica, Lamarck's sp. PI. XL VI, figs. 1,2; PI. XLVII, figs. 1 — 3. 



Galerites cylindricus, Lamarck. Animaux sans Vertebres, torn, iii, 



p. 23, 1816. 



— — Deslonffchamps. Encycl. Method., Zoophytes, 



t. ii, p. 433, 1824. 



Conulus Hawkinsii, Mantell. Geol. Trans., new series, vol. iii, 



part I, p. 208, 1828. 



Galerites canaliculars, Goldfuss. Petrefacta Germanise, vol. i, p. 128, 



pi. xli, 1829. 



1 ' Memoirs of the Geological Survey,' Decade 1, pi. vii, p. 4. 



