﻿DAVIDSONIA. 



may or may not have existed in those unforaminated species which were provided with a 

 straight hinge-line, but 1 have observed imprints of adjuster muscles in those Strophomena- 

 sliaped species which were up to a certain age provided with a foraminal aperture for the 

 passage of a pedicle, as in S( ropltomena analog a, &c. The most important character of 

 Davidsonia resides in the greater portion of the interior of the ventral valve being occupied 

 by two conical elevations projecting more or less beyond the level of the valve, the lateral 

 and frontal portions of these exhibiting five or six semicircular or spiral projecting ridges, 

 which diminish in surface and width as they approach the summit of the cone. In the 

 interior of the smaller valve Prof, de Koninck found two conical hollows which corre- 

 spond with the cones of the attached valve, and were separated by a rounded mesial 

 ridge. 



Various interpretations have been advanced as to the use of these cones, but the most 

 probable one was that they were produced by the mantle, which, pressing on the spiral 

 arms, retained some impressions of their coils, which were transmitted to the shell it was 

 secreting. This view was expressed in the English, French, and German editions of my 

 General Introduction, but I then supposed that the spiral arms were free and unsupported, 

 as in Productus, Strophalosia, &c. ; since that period Prof, de Koninck has discovered 

 two spiral lamellae, which were fixed to the socket-margins of the smaller valve, and 

 formed a few vertical convolutions towards the bottom of the valve, having a somewhat 

 similar appearance to what we perceive in Alrypa, only the two spirals in Davidsonia were 

 not so closely adpressed as in Dalman's genus. 1 The first British example of this genus 

 and species was noticed by myself adhering to a valve of Atrypa aspera in the Jerrnyn 

 Street Museum, and which had been collected many years ago by Mr. Godwin-Austen in 

 the Middle Devonian limestone of the neighbourhood of Newton Abbot. Subsequently, 

 a much larger specimen, attached to Stringocephalus Btirtini, was found by Mr. Vicary in 

 Woolborough quarry, and another by Mr. Champernowne at Dartington, near Totness, 

 Devonshire, where also it occurs attached to corals, &c. On the Continent the shell is 

 very common in the horizon characterized by Calceola sandalina, or Middle Devonian 

 limestone of the Eifel. It occurs also at Chimay in Belgium. 



1 These observations were published by myself in the Xlth number of the ' Geologist' for November, 

 1858, and subsequently translated and published by Prof, de Koninek in the 'Transactions of the Royal 

 Society of Liege, in Belgium, for 1859.' 



