176 



CARBONIFEROUS LAM ULLTBRANCHIATA. 



times, being found at various horizons, from the Calciferous Sandstone Series to the 

 Coal Measures. Nucula gibbosa, Nuculana attenuata, Ctenodonta laevirostris, Portl. sp., 

 occur first of all in the Calciferous Sandstone Series of Scotland, and persist up to 

 the Coal Measure times, occurring in marine bands of the North Staffordshire Coal 

 Measures. 



I drew attention in my paper on the Pendleside Series (loc. eit., p. 380) to the 

 fact that many Lamellibranch species and families which occur in the Calciferous 

 Sandstone Series of Scotland occur at higher and higher horizons for the first 

 time, as the beds pass south, and showed that curves could be constructed showing 

 the earliest known occurrence of each species at a series of localities between Fife 

 and Staffordshire. The name isodietic line was given because it was thought that 



Fig. 2. 



rr 



Isodietic line for 

 Nuculana attenuata, 

 Tfucula gibbosa, and 

 ( i, no&onta laviros- 



Coal-Measures.. 

 Millstone Grits 

 Pendleside Groi 



Carboniferous Lime- 

 stone 



Yoredales & Great 

 Scar Limestone 



Calciferous Sand- 

 stone Series 



--- 



ut 



I ABSENT 



\ 



this condition of things was due entirely to environment. The Carboniferous 

 succession in the north was doubtless laid down much nearer to land than the beds 

 further south, a fact demonstrated by the greater amount of detrital material in the 

 deposits. 



The above tabular diagram (fig. 2) shows the isodietic line for three species, 

 Nuculana attenuata, Nucula gibbosa, and Ctenodonta laevirostris, but it seems that 

 N. attenuata always came in some little time before the others. 



The three genera mentioned above all appear in the Calciferous Sandstone 

 Scries of Scotland, and reappear in that area at many horizons in the Carboniferous 

 Series of that subdivision. 



According to Mr. J. W. Kirkby's tables 1 Nuculana (Leda) attenuata is found 

 3000 to 3800 feet below the Carboniferous Limestone, at a lower horizon than 

 Nucula gibbosa, which comes in from 500 to 2300 feet below that bed. Ctenodonta 



1 'Quart. Journ. G-eol. Soc.,' vul. xxxvi (1880), p. 589. 



