Geological Society. 143 



PROCEEDINGS OF LEARNED SOCIETIES. 



GEOLOGICAL SOCIETY. 



April 27th, 1910.— Prof. W. W. Watts, Sc.D., M.Sc, F.R.S., 

 President, in the Chair. 



The following communications were read : — 



1. ' On the Evolution of Zaphrentis delanouei in Lower Carboni- 

 ferous Times.' By Robert George Carruthers, F.G.S. 



The small simple corals that belong to the gens of Zaphrentis 

 delanouei are of common occurrence in the Lower Carboniferous 

 rocks of Scotland. Their distribution is remarkably sporadic, but 

 it is possible to collect over wide areas of which the stratigraphv 

 is definitely known. A large number of specimens have been got 

 together (some twelve hundred in all), from horizons scattered 

 throughout the sequence. The ontogeny of these specimens has 

 been investigated by means of serial transverse sections. 



The evolutionary changes observed are confined to the disposit ion 

 of the septa, which has influenced the shape of the cardinal fossula 

 in a very marked manner. The external characters, and the spacing 

 and curvature of both septa and tabulae, remain unchanged. 



Zaphrentis delanouei is, typically, a Tournaisian species, and it 

 has a wide fossula, expanded inwardly. When the gens first appears 

 in the Scottish rocks (in the Cementstone Group of Liddesdale) 

 Z. delanouei is the predominant form, but is associated with a 

 mutation (in Waagen's sense) in which the fossula is parallel-sided. 



In the higher limestones of Lawston Linn, another mutation 

 appears, which, for reasons detailed in the paper, is regarded as a 

 sport, or offshoot from the direct line of evolution. 



In the succeeding Lower Limestone Group the gens undergoes 

 further modification. Adults of the two Cementstone species are 

 extremely rare, and the predominant form has a fossula which 

 narrows rapidly to the inner end ; in subordinate association a 

 further mutation is also developed, in which the septa are short 

 and amplexoid. 



In the still higher horizons of the Upper Limestone Group, the 

 last-mentioned mutation becomes predominant, and persists up to 

 the Millstone Grit, where the septa become more amplexoid. 



All these mutations, in neanic lite, have characters seen in adults 

 of the preceding form; tachygenesis is so marked that earlier 

 ancestral traits are rarely seen. 



Mutational percentages are given for many localities in the 

 Carbouiferous Limestone Series of the Central Valley, together with 

 an analysis of the data so obtained. 



Brief diagnoses of the four new species are appended to the paper, 

 together with full locality-lists. 



