382 



BR. W. HIXD AND MR. J. A. HOWE ON THE [^^g. IQOI. 



necessary conditions of deposition and environment for the members 

 of the family Nuculidae did not exist in the south till late on in 

 Carboniferous times, and that the line drawn obli(][uely 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 & 3: one group con- 

 sisting of the Nuculidiie, the other of those genera — Carhonicola, 

 AntJiracoraya, Naiadites — which, from the peculiar erosion of the 

 umbones, are justly considered to have been freshwater dwellers: 

 Naiadites, belonging to a byssiferous group, is chosen because in this 

 case migration was limited naturally by structure and habit. 



The genera Carhonicola and Antliracomija belonging to the Uni- 

 onidse, 2indi Naiadites — a byssiferous genus belonging to the Mytilida), 

 — have long been recognized as characteristic of the freshwater beds 

 of the Coal-Measures, and have a wide horizontal distribution. 

 An examination of the distribution of these genera during Car- 

 boniferous times gives an interesting result. All three genera are 



represented in the old- 

 pig. 3. est Carboniferous rocks 



of Fifeshire: Carhoni- 

 cola by two S})ecieSy- 

 C. antlqua and C. ele- 

 cjans ; Anthracomya h\ 

 A. scotica and another 

 well- developed form 

 closely allied to, if not 

 identical with, A. 

 Adamsii ; and Naia- 

 dites by N. crassa and 

 N. ohesa. 



These genera are, 

 with the exception of 

 N. crassa, absent from 

 the Tuedian Series of 

 x^orthumberland ; but 

 that species occurs in 

 shales in the Carbon- 

 aceous and Fell-Sand- 

 stone Series at Lewis- 

 burn, and a species of 

 the same genus (pos- 

 sibly a dwarf example 

 of N. crassa) is found 

 at Sillsburn in the 

 Redesdale district. Prof. Lebour quotes Anthracosla {Carhonicola) 

 acuta from the horizon of the Redesdale Ironstones, but after 



Isodietic line 

 ioY Carhonicola, 

 Anthracomya, 

 and Naiadites. 



Coal-Measures — 



Millstone Gdts & 



Pendleside Groug; 1 



C a nb on i f e nou s Li roe- 

 stone Serjes 



Calciferous Sand- 

 stone Senies 



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