FROM THE CHLORITIC MARL. 237 



The vent is elliptical, and placed in the middle of the posterior border (fig. 2 d), 

 Jiearer the upper than the under surface (fig. 2 a). 



The apical disc is small, and composed of four ovarial plates, of which the right 

 antero-lateral is much the largest, and supports a spongy madreporiform body. All these 

 plates are perforated (fig. 2/). The ocular plates, five in number; are small, and well 

 wedged in between the ovarials. 



Affinities and Differences. — The regular elliptical elongated form of Pi/rina JDesmou- 

 linsii, with its flattened upper surface and pulvinated base^ distinguish this species from 

 its congeners. I have compared specimens obtained from the Btage Cenomanien of the 

 environs of Tournay, Belgium, with specimens collected from the Chloritic Marl at 

 Chard, and find them to be identical in all their details. It very much resembles Pi/rina 

 ovidtim, which, however, is a smaller form, with a more inflated test, and the elliptical 

 vent is situated near the dorsum. Compare fig. 2 and fig. 3, where the affinities and 

 differences are well shown in the admirable figures in PI. LIV. 



It difiers from Pyrina lavis in having a nari'ower test of a more regular elliptical 

 figure, and wants the inflation of the anterior portion and the tapering of its posterior 

 border. 



Localiiy and StratiyrajMcal Position. — This rare Urchin was collected by Mr. Weistj 

 in the Chloritic Marl, near Chard, and Mr. Pratt obtained another from the same locality 

 and stratum, which has been beautifully figured in the ' Memoirs of the Geological 

 Survey,' Decade v, i)l. vi. The short description was from the pen of my old 

 esteemed friend Dr. Woodward, to whom I forwarded all ray materials when he was 

 engaged in writing the text for the description of pi. vi of that Decade. 



Pyrina ovulum, Lamarck, sp. PL LIV, fig. 3 a — h. 



NucLEOLiTES OVULUM, Lamarck. Anim. sans vert., t. iii, p. 37, 1816. 



— — Beslongchamps. Encycl. Method., t. ii, p. 500, 1824. 



— — Befrance. Die. des Sc. Nat., t. xxxv, p. 213, 1825. 

 Pyrina — Agassiz. Cat. Syst., p. 7, 1840. 



NucLEOLiTES — Desor. Mon. des Galerites, p. 26, pi. v, figs. 35 — 37, 1842. 



— — A(jassizei Desor. Cat. rais. des Rchinides, p. 92, 1842. 



— — (TOrbigny. Prodrome, t. ii, p. 271, 1847. 



— — Morris. Catal. of British Fossils, 2 ed, p. 88, 1854. 



— — Woodward. Mem. Geol. Surv., Decade v, pi. vi, 1856. 



— — Cotteaii. Paleontologie Frangaise, pi. 985, figs. 7 — 11, tome 



vi, p.484, 1855. 



