402 GASTEROPODA OF THE INFERIOR OOLITE. 
preceding species, yet there seems to be a considerable jump between the narrow- 
angled (elongata) and the wide-angled (abbreviata) forms. Moreover, in this 
country Pl. abbreviata is confined to the Humphriesianus-zone, and more 
particularly to the Sauzei-bed or subzone. One of the figures given by d’Orbigny 
of Pl. conoidea, Deshayes (vol. cit., Pl. ecclxxxii, fig. 1), covers this form very 
fairly ; but, as there can be no doubt that the specimens from Dundry and Oborne 
represent the form described by Sowerby as Tvrochus abbreviatus, 1f we are to 
recognise the species at all it should be under the name given by Sowerby. 
Deseription.—The spiral angle is about 65°, and in some cases very slightly 
concave; but the form is usually that of a perfect cone, where the height and 
basal diameter are nearly equal, the latter being usually a little in excess. The 
ornamentation is rather stronger than in most varieties of Pl. elongata, but the 
character of the basal rim and sinus-band is, on the whole, pretty much the same, 
though possibly the nodules of the basal rim have a tendency to become rather 
more complex. The aperture is subquadrate, the columellar lip being only 
slightly oblique, whilst the terminal notch or gutter is barely indicated. 
There is a variety of Pl. abbreviata (see fig. 9) analogous to that which is 
observed in the typical variety of Pl. elongata. Here the ornamentation has a 
tendency to become smoother, especially in the basal carinz. When this is 
carried to excess in very large specimens, we obtain a wide-angled Pl. Ebrayana. 
Relations and Distribution.—Highly ornamented specimens of Pl. abbreviata 
approach Pl. circumsulcata, but in that species the sulcus in the base is usually 
distinct, and where this fails, the concave spire of Pl. circwmsulcata will serve to 
distinguish them. On the other hand, it approaches the conoidea var. of IP 4h. 
elongata. 
Pl. abbreviata is abundant in the Sauzei-bed at Oborne, and possibly occurs on 
the same horizon at Dundry. In the Parkinsoni-zone of Burton Bradstock are 
some small forms, which may be allotted partly to this speciés and partly to PI. 
elongata. 
337. Prevroromarta AcatHa, d’Orbigny, 1850 (= Pl. mutabilis, Deslongchamps, 
pars.) Plate XXXIII, figs. 12 a, 12 5, and 
Plate XXXIV, fig. 6. Var. Sandersii, 
Tawney, Plate XXXIV, fig. 7. Var. un- 
named, XXXIV, fig. 9. 
1850. Pievroromarta AaatHa, d’Orbigny. Prod., i, p. 268. 
1854. = — — Terr. Jur., vol. ii, p. 474, pl. 
ecelxxxui, figs. 1—3. 
