280 GASTEROPODA OF THE INFERIOR OOLITE. 
The aperture in the early stage is very like Purpuwrina, the anterior cana- 
lhieulation being well marked. This feature is also seen in the middle stage, but 
disappears more or less completely in the big shells, where the mouth is sub- 
circular, the shape of the aperture being materially modified, with considerable 
thickening of the inner lip (see especially Pl. XXI, figs. 16, 17, and 18). 
Varieties. — The specimens from the type locality are of medium size. 
Pl. XXI, fig. 15, is probably one of Sowerby’s types with the tuberculations worn 
off the spiral ridges, and the cross-hatchings almost obliterated by usage. 
Fig. 14 from the same locality is in very fair preservation, and probably shows 
the true character of the type form. The cross-hatchings are wide and coarse, 
the posterior spiral is very close to the suture, and there is some tendency to 
tuberculation on the spirals in the base of the body-whorl. 
The variety spinulosa, Minst., Pl. XXI, fig. 15, which I consider synonymous 
with Purpurina Bathis, ’Orbigny, is a large, handsome, and richly ornamented 
form, which occurs rarely in the Concavus-bed.at Bradford Abbas. 
The variety to which I have given the name abbas, Pl. XXI, figs. 16, 17, 18, 
and Pl. XXII, fig. 1, is extremely common on the same horizon and at the same 
place. From well-preserved specimens of this most abundant form we gather 
that, although the spire is pointed on the whole, yet the apex is obtuse; the 
earlier whorls are extremely angular, and the third spiral is seen to be developed 
at a very early stage, already receding and less prominent than the others. In 
the more mature whorls of this variety the crenulations or tuberculations on 
the spirals become very faint, and in the very large shells disappear altogether. 
On the whole the var. abbas is rather more slim in shape and more delicate in 
ornamentation, though attaining to a much greater size than the type variety. 
Relations and Distribution.—We may well regard this as nothing more than a 
modified descendant, on a higher horizon, of Amb. capitanea. Besides the 
differences already indicated, it may be worth while to point out that the spirals 
in the case of Aiberleya ornata are but slightly tuberculated as compared with 
those of Amb. capitanea. 
In Dorsetshire Amb. ovnata is essentially a fossil of the Concavus-zone, and is 
probably the most abundant fossil on that horizon at Bradford Abbas. At 
Dundry one cannot say on what horizon it occurs, but probably below the Hum- 
phriesianus-zone. Itis rare in the Cotteswolds, where specimens are usually small 
and poorly preserved. Small forms of Amb. ornata, showing a slight tendency to 
produce an intermediate keel in the body-whorl, occur towards the base of the 
Inferior Oolite at Lincoln. There are also small specimens in the Dogger at 
Blue Wyke, which may either be immature forms of Amb. ornata or varieties of 
Littorina (Turbo) Phillipsii. 
