DAVIDSON — ON SOME FOSSIL BEACHIOPODA. 
465 
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 semi-circular 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 correspond 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 was 
that they were produced by the mantel, 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 oral arms were free and unsupported, as 
in Productus, Stro2)homena, ficc. ; but 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 con- 
volutions towards the bottom of the valve, having a somewhat 
similar appearance to what we perceive in Atrypa, only the two 
spirals in Bavidsonia were not so closely adpressed as in Dalman's 
genus. 
Such is the state of our present information regarding the spiral arms 
and their calcareous support, and although much has been ascertained, 
much more remains still to be discovered or sought for before we 
shall be able to furnish complete restorations of the interior of 
several of the proposed sub-genera and species ; but 1 feel confident 
that much of the still deficient information will ere long be acquired, as 
several zealous and able palaeontologists are using all their efforts to 
complete the history of a class which has afforded so much interest in 
every respect. 
We now conclude this communication by furnishing a catalogue of 
all the British Carboniferous species at present known to have been pro- 
vided with spiral appendages for the support of the oral arms. This 
investigation has occupied much of my time during the last two years, and 
I found that out of about 125 so-termed species, said to have been found 
in the Carboniferous strata of Great Britain, not more than forty or 
forty-one could be retained, all the remaining number being made up of 
synonyms of species not positively known to occur in Great Britain, or 
of Carboniferous shells erroneously referred to Devonian species, as 
