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 PI. 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., PI. ccclxxxii, 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 Trochus abbreviatus, if we are to 

 recognise the species at all it should be under the name given by Sowerby. 



Description. — 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 PI. 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 PI. abbreviata (see fig. 9) analogous to that which is 

 observed in the typical variety of PI. elongata. Here the ornamentation has a 

 tendency to become smoother, especially in the basal carinas. 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 circumsulcata will serve to 

 distinguish them. On the other hand, it approaches the conoidea var. of PL 

 elongata. 



PL abbreviata is abundant in the Sauzei-bed at Oborne, and possibly occurs on 

 the same horizon at Dundry. In the P arhinsoni-zor\e of Burton Bradstock are 

 some small forms, which may be allotted partly to this species and partly to PL 

 elongata. 



337. Pleueotomaeia Agatha, d'Orbigny, 1850 (—PL mutabilis, Deslongchamps, 



pars.) Plate XXXIII, figs. 12 a, 12 b, and 

 Plate XXXIV, fig. 6. Var. Sandersii, 

 Tawney, Plate XXXIV, fig. 7. Var. un- 

 named, XXXIV, fig. 9. 



1 850. Pletjrotomaeia Agatha, d'Orbigny. Prod., i, p. 268. 

 is.")] — Terr. Jur., vol. ii, p. 474, pi. 



ccelxxxiii, figs. 1 — 3. 



