178 



CARBONIFRROUS LAMELLIBRANCHIATA. 



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

 was limited naturally by structure and habit. 



The genera Carbonicola and Anthracomya belonging to the Uniomdae, and 

 Naiadites — a byssiferous genus belonging to the Mytilidae — have long been recog- 

 nised 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 Carboniferous times gives an interesting result. All three genera are 

 represented in the oldest Carboniferous rocks of Fifeshire : Carbonicola by two 

 species, G. antiqua and G. elegans; Anthracomya by A. scotica and another well- 

 developed form closely allied to, if not identical with, A. Adamsii ; and Naiadites 

 by N. crassa and N. obesa. 



Fig. 3. 





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These genera are, with the exception of N. crassa, absent from the Tuedian 

 Series of Northumberland ; but that species occurs in shales in the Carbonaceous 

 and Fell- Sand stone Series at Lewisburn, and a species of the same genus (possibly 

 a dwarf example of N. crassa) is found at Sillsburn in the Redesdale district. Prof. 

 Lebour quotes Anthracosia (Carbonic <>/</) acuta from the horizon of the Redesdale 

 Ironstones, but after examination of the specimen I am not able to recognise that 

 it belongs to that genus. 



Farther south, in the Yorkshire dales, the three genera have not been found 

 either in the Great Scar or in the Yoredale Series. 



Still farther south, in Staffordshire and Derbyshire, these genera only come in 

 at the base of the Coal Measures, but they are each represented by numerous species. 



If the horizons at which n large number of the marine fossils of the Calciferous 



