48 



MONOGRAPH OF DURA DEN. 



combination of the successive tribes into distinct faunae and 

 florae during each period of change, and to gain some insight, 

 if possible, into the causes, or even the circumstances, under 

 which the changes themselves may have taken place. Our 

 small contribution to this interesting department of Palseon- 

 tologj comprises a group of creatures, which, in structure and 

 form, have many affinities to each other, and comparatively 

 few to the occupants of the waters in succeeding times, or 

 to any now living ; and, in point of geographical distribu- 

 tion, our field of review is of very . limited extent, covering 

 only as yet a few square yards of fossiliferous sandstone, lying 

 on the confines of the Devonian and Carboniferous epochs of 

 organic life. 



Sphonopteris Affinis. 

 (Carboniferous Sandstone of Dura Don ) 



