40 



fig- 3)> from the Dakota Group* which shows a lobed terminal 

 leaflet. This species has a petiole that is partly winged, suggest- 

 ing Rhus copallina L. 



Thus, there would seem to be two species of fossil Rhus leaves 

 whose method of development was similar to that of the living 

 Rhus glabra L. and Rhus copallina L. and, therefore, basifugal. 



Another species which presents an interesting example of 

 arrested leaf development is Negundo triloba Newb.f (pi. A, fig. 

 4, a) from the Fort Union Group. This leaf has a lobed terminal 

 leaflet that is almost wholly split off, suggesting a basifugal 

 tendency such as can be found in the living Acer Negundo (pi. A, 

 fig. 4, b). 



In the same work (pi. 30, fig. 2) a specimen of the fossil species 

 Sapindus membranaceus Newb., is figured, which shows a lateral 

 leaflet completely split from the terminal one (pi. B, fig. i , a) . An 

 investigation of the specimens of Sapindus in the herbarium of the 

 New York Botanical Garden revealed some leaves of Sapindus 

 saponaria L. in which the terminal leaflet had split to form a new 

 lateral (pi. B, fig. i, ^). Here again is a case of a living form 

 and a fossil form of apparently the same genus developing their 

 leaves in the same way. 



In Fontaine's Potomac or Younger Mesozoic Floral several 

 leaves are figured whose affinity to Sapindus is indicated by the 

 generic name Sapindopsis . Certain leaves of this genus, belong- 

 ing to two different species {S. variabilis Font, and 5. magnifolia 

 Font.) exhibit lobed terminals in various stages of development 

 (see pi. B, figs. 2, a and 2, b). In fact, a short though incomplete 

 series of the figured leaves of Sapindopsis variabilis Font, might 

 be arranged to show the successive steps in the formation of the 

 lateral leaflets from the terminal leaflet. 



A further search of paleobotanical literature and of duplicate 

 specimens of fossil plants would doubtless disclose many other 

 interesting examples. 



* Mon. U. S. Geol. Survey 17: 154. pi. 57, fig- 2. 1892. 



t Newberry, Mon. U. S. Geol. Survey 35: 115. pi. 31, fig. 5- 1898. 



X Mon. U. S. Geol. Survey, 15: 297, pi. 151, figs. 2, 3; pi. 152, figs. 2, 3; 

 pl. 153, fig- 2; pi. 154, figs. I, 5; pl- 155, fig- 6: 298, pi. 151, fig. i; pi. 132. 

 figs. 1-4; pl- 153, fig- 3; pl- 154, figs. 2-4; pl. 155, figs. 2-5. 1889. 



