AMBERLEYA. 289 
Description, Section A : 
Length of adults (Trochus bisertus) . . 25—30 mm, 
Length of body-whorl to total height . . 54: 100. 
Spiral angle : yn iiae 
Shell conical, trochiform. Spire pointed, with probably an obtuse apex; 
number of whorls about eight. The extreme apical whorls are smooth, convex, 
and without ornament ; remaining whorls flat or slightly concave; sutural gutter 
very deep. The ornaments consist of two finely tuberculated circlets or girdles 
of nearly equal strength, one close to the posterior margin, the other, which 
forms a slightly salient keel, is situated near the anterior extremity. The whorls 
are richly ornamented by a system of deeply-cut axial striz, which add much to 
the beauty of these shells. 
In the body-whorl a similar system of ornamentation prevails, and the axial 
striz are continued down to a third circlet, which is tuberculated. The base 
varies from moderately full in the early stage, where the columellar lip is straight 
and pointed at the extremity (fig. 13), to rather flat in the mature stage, where 
the aperture is subrhomboidal or subcircular (fig. 14); it carries four or five 
spirals, which are nearly plain. 
N.B.—Specimens from Drympton, such as the one figured, have a flatter base 
and more trochiform aspect than those from the Dogger. 
Section B.—The specimen of Littorina wnicarinata (Pl. XXIII, fig. 1) has been 
somewhat distorted by compression, which has the effect of increasing the apparent 
width of the spiral angle. The points in which the wnicarinata-variety differ from 
Section A are mainly those of ornamentation. The lower or keel girdle prepon- 
derates greatly over the other, which latter in some specimens is almost effete. 
This has the effect of making the shell more unicarinate and less trochiform in 
outline, and in the adult forms (fig. 2) recalls the Hucyclus-section. 
Relations and Distribution.—Amberleya biserta and its varieties seem to form a 
group somewhat isolated. My reasons for not regarding it as a Trochus were 
given in the ‘Geological Magazine.’ Continental authors do not seem to have 
noticed it; nor can I find that d’Orbigny, who generally quotes Phillips’ 
species, mentions it in the ‘ Prodrome.’ 
In England it is essentially a fossil of the Opalinus-zone or lower part of the 
Murchisone-zone, occurring sparingly in the Dogger at Blue Wyke, and in the 
Dorset-Somerset district at Drympton and Haselbury. Drympton is, on the 
whole, the best locality for these beautiful trochiform Amberleyas, and there are 
indications there also of the unicarinate variety. 
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