100 EOCENE MOLLUSCA. 
deep umbilical cavity beneath ; the six or seven volutions of which the shell is formed, 
are flat, or nearly so, above; convex below; and each but slightly embracing the 
preceding volution; the apex is concealed by the involution of the whorls. A clearly 
defined and rather acute angle, almost forming a keel, runs round the periphery of 
the shell, and separates the upper from the under disc. This angle is always very 
conspicuous in young shells; but as they approach maturity, it becomes more and 
more obtuse, and frequently altogether disappears; the whorls then assume a trans- 
versely oval form, approaching nearly to that presented by P. rotundatus. On the 
under surface, the inner margins of the whorls are bent rather suddenly towards the 
preceding whorl, and present, in consequence, an obtuse angle, which runs round 
and defines the umbilicus. Transverse lines of growth are very conspicuous; and 
frequently the surface of the shell also presents more or less numerous concentric 
raised lines, some of which are larger and more prominent than the rest. The 
aperture is sub-trigonal, slightly impinged upon by the preceding volution, and very 
oblique. 
This species presents a general resemblance to P. rotundatus; but, even when the 
characteristic angle has become obsolete, it may easily be distinguished by the greater 
breadth, and the flatness of the upper surfaces, of the whorls, the much larger concavity 
on the under side of the shell, and the subtrigonal and more oblique aperture. From 
P. discus it is separated by the more compressed form of that species, caused by the 
greater width of the whorls, and the comparative flatness of their under sides ; and in 
that species the whorls are more embracing than in this. 
Size-—Diameter, 1 inch and 6-10ths. 
Localities—Headon Hill; Hordwell. 
No. 52. PLANoORBIS ROTUNDATUS. Brard. Tab. XV, fig. 4 a—e. 
PLANoRBE ARRONDI. Brard. 1809. Ann. du Mus,, vol. xiv, p. 433, t. 27, figs. 19, 20. 
PLANORBIS ROTUNDATUS, Brogn. 1810. Ib., vol. xv, p. 370, t. 22, figs. 4, 5. 
—_ —_ Tb. 1811. Jour. de Phys., vol. Ixxii, p. 419, figs. 4, 5. 
— SIMILIs. Fér. 1814. Mem. geol., p. 61, No. 1. 
= RoTUNDATUS. Fér. 1814. Ib., No. 2, var. a. 
= = Desh. 1824—37. Desc. des coq. foss., vel. i, p. 83, t. 9, 
figs. 7, 8. 
— a Desh. 1824—37. Encycl. Meth. Vers., &c., vol. i, p. 778, No. 2. 
— — Bowd. Elem. of Conch., t. iv, fig. 7. 
— — Lyell and Mur. 1829. Sur les depots, &c. du Cantal. 
— —_ Bouillet. Desc. hist. et scient. de la Haute Auver., p. 18, figs. 6, 7. 
— ~ Ib. 1836. Cat. des coq. viv. et foss. de l Auver., p. 115, No. 2. 
—_ —_ Gratel. 1838. Cat. des deb. foss., &c., du bassin de la Gironde, 
i p. 33, No. 102. 
= = Desh. 1838. Lam. Hist. Nat., 2d edit., vol. viii, p. 394, No. 1. 
