TROCHUS. 375 



312. Trochus Ddryanus, d'Orbigny, 1852. Plate XXXI, fig. 12. 



1852. Teochus Duryanus, d'Orbigny. Terr. Jur., vol. ii, p. 280, pi. cccxiv, 



figs. 12—15. 



Description : 



Height . . . . .7 mm. 



Width . . . . .6 mm. 



Spiral angle . . . . .52°. 



A single specimen from Grove (Pa/rkmsoni-zone), having the above dimensions, 

 so nearly accords with the description and figures of d'Orbigny that we seem fairly- 

 safe in making the identification. 



This little shell has no umbilicus ; otherwise it seems to vary from its near 

 relative, Trochus duplicatus, in a direction precisely opposite to that of Trochus 

 angulatus. 



The Sitbduplicatus-group. 



There is a varied series of trochiform shells in the Lower Division of the 

 Inferior Oolite, mostly towards the Lias-boundary, which it is convenient to focus 

 under " Turbo " sub duplicatus, d'Orbigny, as the primary division, even if we 

 indulge in varietal names. 



313. Trochus subduplicatus, d'Orbigny, 1850. Plate XXXI, figs. 13 and 14. 



1850. Turbo subduplicatus, d'Orbignt/. Prod., 1, p. 248. 



1852. — — — Terr. Jur., vol. ii, p. 339, pi. cccxxix, 



figs. 1 — 6 (notably figs. 4, 5, and 6). 



Bibliography, 8rc. — When d'Orbigny, in the " Terrains Jurassiques," placed 

 Trochus duplicatus, Sow., at the head of the synonymy of " Turbo " subduplicatus , 

 d'Orb., that author appears to have forgotten that he had already (p. 275, 

 pi. cccxiii, figs. 5 — 8) accepted Sowerby's species, which he described and figured 

 with his usual fidelity. If anyone doubts the essential difference between 

 T. duplicatus, Sow., and T. subduplicatus, d'Orb., let him refer to the figures and 

 descriptions in d'Orbigny's own work. Moreover T. duplicatus, Sow., and 

 T. subduplicatus, d'Orb., besides being very distinct in form, occur on quite 

 different horizons. 



