1902] NOTES ON THE PHYLOGENY OE LIRIODENDRON 53 



forms referred io L. ^simplex and L. primaevitm Newb., showing 



every gradation in form from the elongated notched leaves 



through the typical Z. simplex to the broad forms of L, prima eviim, 



which show a tendenc}^ to develop fouf lobes, a stage reached 



in Z. Meekii Heer. The typical forms of L. primaeviun are later 



in point of time than Z. simplex, being found in the Dakota group, 



which forms the lowermost layers of the Upper Cretaceous; 



while Z. simplex begins in the Aniboy clays, long thought to be 



the same age as the Dakota, but now referred to some of the 



later Potomac series (Albirupean) of the Lower Cretaceous. 



However, many of the forms which have been referred to *Z. 



simplex are intermediate between it and L. primaeviim, there 



being no very clear lines of demarkation among Liriodendropsis 



angtistifoha, Liriodendro7i simplex, L. primaevum, and L. Meekii; 



each being a modification of the preceding through insensible 



gradations. It is but a step from some forms of L. priinacvum 



to L. Meekii, and we have among our collections of L. Ttdipi- 



fera many leaves that approximate those of L. Meekii in 

 shape. 



The fourth type of this group represents the other extreme 

 of form, and is somewhat removed from the preceding three. 

 It preserves the pointed apex of the original ancestor, which 

 gradually broadened until it developed an obtuse basal lobe on 

 each side. This form is represented among our known fossil 

 torms by Liriqdendron semialatum Lesq., and among leaves of L. 

 Tulipifera by several specimens. L. semialatum is found in the 

 Dakota group, and would have had ample time to develop into 

 a broader leaf witH basal lobes during the long period of the 

 Lower Cretaceous when the Amboy clays were being deposited 

 along our eastern coast. The small leaves from the Upper Cre- 

 taceous of Vancouver island, described by Dawson as L. siicredens, 

 if they belong to this genus would be the natural descendants 

 of Z.. semialatum. 



If 



we call the theoretical oblong-lanceolate form stage i, 



the preceding five closely related leaf types fall naturally in a 

 gi-oup which may be called stage 2. 



