DISTRIBUTION. 177 



Isevirostris is not mentioned by him, and we have been unable to give the exact 

 point at which it first conies in, but in the upper part of the Calciferous Sandstone 

 Series of Fife all these fossils arc found together. They never occur in the pure 

 white or grey limestones, only in the shales between them. 



In the West of Scotland these species are well represented in the shales of the 

 Carboniferous Limestone series. Mr. J. Smith, of Kilwinning, informs us thai 

 they have not, been found below the " shale " under the lowest " post " of the Lower 

 Limestone Series. 



The Calciferous Sandstone Series in Eskdale, however, does not seem to possess 

 these species, though they all come in in the shales associated with the limestones on 

 the horizon of the Hurlet Limestone. Farther south, in Northumberland, Nuculana 

 at l<'ii until, Nucula glbbosa, and Otenodonta Isevirostris are absent in the Tuedian 

 series ; N. attenuata comes on alone in the Carbonaceous division, but N. gibbosa 

 is found with it in the shales of the Calcareous division above at several horizons. 



Still farther south, the lowest horizon at which we have been able to obtain 

 Nuculana attenuata and Ctenodonta Isevirostris in the valley of the Eden is in shales 

 presumably above the Underset Limestone. They probably do occur somewhat 

 lower, however, for we have obtained Ctenodonta Isevirostris in shales below the 

 Hardraw Scar Limestone, although at present Nuculana attenuata and Nucula 

 gibbosa are not known so low down in this locality. Farther south, again, in beds 

 presumably immediately above the main mass of limestone at Whitewell, Nuculana 

 attenuata and Ctenodonta Isevirostris appear, the latter being found at more than 

 one horizon in the Pendleside Group. 



Continuing in a southerly direction, we find in the Marsden Valley, at Eccup, 

 near Leeds, and Congleton Edge (Cheshire), the lowest horizon for Nucula gibbosa 

 and Ctenodonta Isevirostris in the upper part of the Pendleside Group and Shales 

 below the Third Grit; while these shells are found at one or more horizons in the 

 Coal Measures of Lancashire and North Staffordshire. Nuculana attenuata has 

 disappeared, but its place has been taken by Nuculana stilla. 



This peculiar distribution of allied forms of shells is very striking, and seems to 

 us to point conclusively to the fact that the necessary conditions of deposition and 

 environment for the members of the family Nuculidre did not exist in the south till 

 late on in Carboniferous times, and that the line drawn obliquely across the strata 

 from the lower part of the Calciferous Sandstone Series to the Millstone Grits of 

 the Midlands represents an isodietic line for this family, which is exact for the 

 individual species representing them. It will be seen that similar lines which have 

 an almost identical curve can be constructed for other groups. Details of two 

 groups are given in figs. 2 and 3, one group consisting of the Nuculidaj, the other of 

 those genera — Carbonicola, Anthracomya, Naiadites — which, from the peculiar 

 erosion of the umbones, are justly considered to have been freshwater dwellers; 



