380 



DR. W. HIND AND MR. J. A. HOWE ON THE [Aug. I90I, 



sub-zone in the lower part of the larger zone of Posidoniella loevis 

 and Avieulopecten papyraceiis. 



The vertical distribution of the Pendleside fauna shows that the 

 generally accepted subdivision of the Upper Carboniferous beds had 

 no palseontological basis, and that on these grounds it is impossible 

 to separate the Pendleside Group, Millstone Grits, and Lower Coal- 

 Measures one from the other ; such a subdivision is purely litho- 

 logical and local. Some species of this fauna are even found at 

 one or two horizons in the Middle Coal-Measures : as, for example, 

 Ctenodonta IcEvirostris, Avieulopecten papyraceus, and others, in the 

 I^orth Staffordshire Coalfield, and near Ashton-under-Lyne. 



While working at a Monograph of British Carboniferous Lamel- 

 libranchs, one of us found that the peculiar distribution of certain 

 genera and species at widely different horizons in different areas was 

 verj^ striking. As evidence accumulated, it became certain that 

 this distribution depended on similarity of conditions of 

 deposition. It seems to us that by taking some fossil, or group 

 of fossils, whose habits can be compared with some living repre- 

 sentatives, and ascertaining at what horizons they occur in different 

 localities, it may be possible to construct isobathy metric lines in 

 the series of rocks or, at least, to draw lines in them connecting 

 points of similar conditions of deposition ; such lines we propose to 

 call is die tic. As an example, we may take the Nuculidae, a 

 family which is represented in Carboniferous times by the genera 

 Nuculana, Nucida, and Ctenodonta. The habits of recent members 

 of this family and the depths at which they live are well known ; 

 S. P. Woodward gives the distribution of Nucula at from 5 to 100 

 fathoms. 



The following tabular diagram (fig. 2) shows the isodietic line for 



Pig. 2. 



Isodietic line for 

 Nuculana at- 

 tenuata, Nucula 

 gibbosa, and 

 Ctenodonta 

 Icevirostris. 



Coal-Measures, 

 Millstone Grits 



Pendleside Grou£. _ 



C arbonlf erous Lim;e 

 s.tone 



YoredaIes& Great 

 Soar Limestone 



Caiciferous Sand- 

 stone Series 



