384 DR. W. HIND AND MR. J. A. HOWE ON THE [^Ug. IQOly 



limestones, but has not been found in the Great Scar Limestone 

 below. In the area where the southern rock-types are found it is- 

 absent in the great mass of limestone, but occurs in the Pendleside 

 Shales, and at one or two horizons in the Coal-Measures of North 

 Staffordshire. 



The byssiferous and always marine genus Myalina is represented 

 in the Calciferous Sandstone Series by M. suhlamellosa, M. Flemingi,- 

 M. Verneuilii, 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 ; Parallelodon bistriatus and P. semi- 

 costatus ; ProtoscJiizodus axiniformis ; Sanguinolites anyustatus, 

 S. striatolamellosus, and S. plicatus ; AUorisma maxima, A. sidcata, 

 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 

 horizons. Isodietic lines, therefore, in no way indicate 

 time, but simply physiographical conditions, and in 

 this sense are also life-zones. 



It is to be seen from the fossil-lists in Appendices A & B (facing 

 p. 402) that some few of the brachiopoda could accommodate 

 themselves to an altered environment, but that at the same time 

 the restriction of certain well-marked species to certain horizontal 

 zones is very definite. 



These tables clearly demonstrate that a similar set of conditions 

 slowly passed along from north to south. Either the depth of 

 the sea was shallowed by the accumulation of sediments to such 

 an extent that sublittoral and byssiferous species found in it their 

 necessary environment, or the sea was shallowed by elevation 

 of the bottom : most probably the former. During the deposition 

 of the Calciferous Sandstone Series in Fife, estimated as 3800 feet 

 thick, marine limestones (averaging only about 50 feet) were laid 

 down, containing practically the same faunas at several different 

 horizons, and demonstrating that the amount of sedimentation was 

 about equal to the amount of depression. The overlying series 

 (the Carboniferous Limestone Series of Fife) comprises a much 

 larger number of marine beds, and contains pure limestones of 

 considerable thickness, showing temporary absences of detrital 



