DISTRIBUTION. 170 



Sandstone Series of Fife occur in other districts were noted, similar isodietic lines 

 would be shown. In the case of the Lamellibranchs, which we have chosen for the 

 investigation (1) because they are fairly well known, (2) because we were able to 

 distinguish the species with some approach to accuracy, and (3) because in the 

 adult stage they do not possess active means of migration, the isodietic line for the 

 whole Lamellibranch fauna of the Calciferous Sandstone Series lies within very 

 narrow limits. It is practically identical with that of the Nuculidce : that is to say, 

 as one passes southward, a large part of the fauna of the Calciferous Sandstone 

 Series occurs at continuously higher horizons, showing the gradual southward 

 spread of similar conditions of environment. Many of the Lamellibranchs of the 

 Calciferous Sandstone Series, preferring muddy and turbid waters, evidently could 

 not live in the clear waters where limestones were accumulating. Thus it may be 

 inferred that as Carboniferous times went on, the influence of the land was felt 

 farther and ever farther south, as is shown by the tendency to interruption of the 

 deposition of limestone by detrital shales and sandstones, and eventually the complete 

 cessation of the formation of pure limestones, even in the area of maximum deposition. 



With regard to Ptevinopectcn papyraceus, which we have chosen as a zonal 

 form, it is interesting to note that it occurs at a lower horizon in Scotland than it 

 does in England. 



It is found in shale at East Kilbride, 2^ feet above the Calderwood Cement- 

 Stone at Glebe Quarry, which is supposed to belong to the Lower Limestone Series 

 of Scotland ; but it seems possible that the beds really belong to the Upper Lime- 

 stone Series, for lithological and palasontological reasons. In Northumberland this 

 species does not seem to go below the base of the Coal Measures, but it occurs in the 

 IVndleside Group and passes up to the Coal Measures in the Northern Midlands. 



The byssiferous and always marine genus Myalina is represented in the 

 Calciferous Sandstone Series by M. sublamellosa, M. Flemingi, M. Vemeuilii, and 

 M. lamellosa. In Northumberland this genus comes in the Carbonaceous division, 

 in Northern Yorkshire it is found in the middle of the Yoredale Series, in Southern 

 Yorkshire in the Millstone-Grit Series, and in Derbyshire in the top-beds of the 

 Limestone, while in Cheshire it occurs in the Pendleside Group of Congleton Edge. 



Isodietic curves similar to those above described could be traced for Edmondia 

 unioniformis, E. rudis, E. sulcata, E. M'Coyii, E. laminata, and E. scalaris; Paral- 

 lelodon bistriatus and P. semicostatus ; Protoschizodus aodniformis ; Sanguinolites 

 angustatus, 8. striatolamcllosus, and 8. plicatus ; Allorisma maxima, A. sulcata, and 

 others. 



These isodietic curves, it will be observed, cut the zonal lines obliquely, and in 

 no way run parallel to them ; and this must necessarily always be so, for as the 

 littoral beds of a slowly sinking or rising area advance or retreat, migration of 

 faunas must take place along lines which intersect the other life-zones at different 



