HIND : CARBONIFEROUS ROCKS OF THE PENNINE SYSTEM. 455 
line extended to Scotland comes at tlie top of the Upper Limestone 
series, and it is here that the great paleontological break occurs. 
T pointed out in my paper (op. supra cit., page 3(S0) that certain 
genera and species are found at much lower horizons in the Carbon- 
iferous series in the north than in the south, and cited as good ex- 
amples the various species of the family Nuculidae. Nuculana 
attenuata and Nucula gibbosa occur in the Calciferous Sandstone 
series of Fife, far below the Hurlet Limestone series, or the base of 
the Lower Limestone series, and are also to be found recurring in 
calcareous shales as high as the Upper Limestone series. Nuculana 
attenuata appears to have come into the area some time before Nucula 
gibbosa. In the west of Scotland these species have not been found 
below the Beith Limestone series, the equivalent of the Hurlet. They 
appear to be absent in the Calciferous Sandstone series of Eskdale 
and in the Tuedian series of Northumberland, but attenuata 
appears in the Carbonaceous division, and both shells are plentiful 
at various horizons in the calcareous division. 
Still further south, the lowest horizon in the Eden valley at 
which these two species have been found, is the shale over the Under- 
set Limestone. Still further south the lowest horizon for N'. attenuata 
is in the shales of the Pendleside series at Whitewell, while in South 
Yorkshire Nucula gibbosa has not been found below the shales 
below the Third Grit at Eccup and Congleton Edge (Cheshire), and 
in North Stafltordshire it occurs at one or two horizons in the Coal 
Measures. These two species seem to occur at higher and higher 
horizons as the beds pass south. We have proposed the term IsoDlETiC 
for the line drawn across the strata representing the migration of 
these shells, which denotes a life zone due to conditions rathei' than 
to time (Fig. 2.). 
We have pointed out that other species and many families appear 
to have migrated slowly south, and for further details would refer to 
pages 380-385 of that paper. 
The majority of the lamellibranchs which occur in the Calciferous 
Sandstone series of Fife obey the same law. As I pointed out, this 
law obtains for byssiferous as well as for free lamellibranchs. 
