THE GEOLOGIST. 



pear to radiate from aline of imaginary centres, whicli are coincident 

 with the longest axis of the flow ; the inner circumference of these 

 radiations being defined by the upper surface of the lava-bed, and 

 hence the upright planes of columnar crystallization strike at right 

 angles downwards from what must have been the primary cooling 

 surface of the mass, that surface from the first having been shghtly 

 depressed in its centre. 



5th, The columns which form the " Giant's Causeway" exhibit a 

 peculiar beauty and accuracy of form, being in every respect more 

 symmetrical than those of any of the other columnar basalts of the 

 district, with the one exception of the " Organ-"columns, and on 

 this account alone an observer would be led to identify them as be- 

 longing to the same bed. 



THE CARBOKLFEROUS SYSTEM IN SCOTLAOT) CHARAC- 

 TERIZED BY ITS BRACHIOPODA. 



By Thomas Davidson, Esq., F.R.S., F.G.S., Hon. Member of the 

 Greological Society of Glasgow, etc., etc. 



(Continued from vol. u.,jpage 4^77.) 



Genus Spiripera. Sowerby. 1815. 



The shells of which this genus is composed differ much in their external 

 shape and appearance, lience the great difficulty of correctly determiiiing the 

 hmits of certahi species. The character of this extinct genus are now so well 

 miderstood that it is scarcely necessary to make any fm'ther allusion to the 

 subject ; but we may briefly repeat, for the sake of the less initiated, and in 

 order to shorten the descriptions of the various species, that all possess a 

 straight liinge-line, and a triangular or sub-parallel area, wliich is divided by a 

 triangular fissure, this last being more or less covered, or contracted, by the 

 means of one or two curved plates, to which the term pseudo-deltidium has 

 been appKed. The pseudo-deltidium is rarely presen-ed in the carboniferous 

 specimens, but did certainly exist in the perfect or hving mdividuals. The valves 

 are articulated by the means of cuived teeth developed on either side of the 

 fissure in the ventral valve, and which fit into corresponding sockets in the 

 opposite or dorsal one. In the larger valve the teeth ai-e supported by vertical 

 plates of greater or lesser dimensions, and in the space between these on the 

 bottom of the shell are situated the muscular impressions. The adductor, or 

 occlusor muscle leaves a narrow mesial oval-shaped scar, and on either side are 

 situated the cardinal, or divaricator muscular unpressions. In the interior of 

 the smaller, or dorsal valve there exists two large corneal spnal coils, which 

 nearly fill the interior of the shell, the ends being directed outwardly towards 

 the cardinal angles, while the bases of the hollow conical spues nearly meet at 



