﻿AMBERLEYA. 



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Description, Section A : 



Length of adults (Trochus bisertus) . . 25 — 30 mm. 



Length of body-whorl to total height . . 54 : 100. 



Spiral angle . . . .55°. 



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 stride, which add much to 

 the beauty of these shells. 



In the body-whorl a similar system of ornamentation prevails, and the axial 

 strise 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 unicarinata (PI. 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 unicarinata-v&riety 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 Eucyclus -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-zonQ or lower part of the 

 Murchisonae-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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