110 Mr Seward, Notes on the [Mar. 9, 



so that ERO is the plane bisecting the external angle between 

 the faces. Thus as IT = D, NN' = A, we have 



sin ^ D _ sin (i — r) 

 sin \A sin r ' 



where i, r are the angles of incidence and refraction IN, UN. 



It is easy to see by geometrical construction that the right- 

 hand side of this formula increases with r. It then follows that D 

 is least when r is least, that is, when R is at : thus the absolute 

 minimum of deviation belongs to the ray passing in the principal 

 plane of the prism. 



Monday, March 9, 1896. 



Professor J. J. Thomson, President, in the Chair. 



Mr R. Hargreaves, late Fellow of St John's College, was elected 

 Fellow of the Society. 



The following Communications were made to the Society : 



(1) Notes on the Geological History of Monocotyledons. By 

 A. C. Seward, M.A. 



In discussing the relative antiquity of the two great divisions of 

 Angiospermous plants, the statement is commonly made that 

 Monocotyledons occur in older rocks than those containing the 

 earliest traces of Dicotyledons. 



The evolution of the Angiosperms and the lines of develop- 

 ment of Monocotyledons and Dicotyledons, are among the most 

 interesting botanical problems ; and it is of the utmost importance 

 to critically examine the evidence afforded by paleobotany as 

 to the order of appearance of these two classes of plants. 



Fragments of parallel-veined leaves and pieces of stem struc- 

 tures have frequently been described as species of Monocoty- 

 ledonous genera; such determinations, however, as are based on 

 mere external and superficial resemblance, cannot as a rule be 

 accepted as trustworthy. 



The cone-like fossils from Jurassic and Lower Cretaceous rocks 

 described under the generic name Kaidacarpon, and referred to 

 the Pandanaceai on the grounds of a similarity of form and 

 structure to the inflorescence of recent genera of that family, are 

 most probably Araucarian cones, and should be placed in the 

 genus Araucarites. 



