GEOLOGICAL HISTORY OF THE WALNUTS 239 



is from deposits as late as the early Pleistocene but this is not 

 based upon positively identified material. 



In Europe the records of Pterocarya commence with the Oli- 

 gocene. The Tertiary species are numerous and widespread, the 

 abundant Pterocarya denticulata Heer being found from Bohemia 

 and Transylvania through Germany, Switzerland and England 

 northward to western Greenland. This widespread species which 

 continues unabated throughout the Pliocene period is thought 

 to be the direct ancestor of the existing Pterocarya caucasica. 

 There are at least five additional Miocene species. 



The Pliocene species are numerous and abundant and are 

 found all over southern Europe being especially common along 

 the elevated shores of the extended Mediterranean sea, in the 

 plateau region of central France, and in the Appenines of Italy. 

 The still existing Pterocarya caucasica makes its appearance in 

 the plateau region of central France at this time where it is repre- 

 sented by both leaves and the characteristic fruits. It still grew 

 in Netherlands in the early Pleistocene according to Dubois, 

 but was apparently exterminated during the glacial period. It 

 is also known from the Altai Mountains of central Asia in deposits 

 of this age. In figure 4 the known range of the fossil species 

 is shown by vertical lining. It seems obvious from the distri- 

 bution of the ancestral forms and the very circumscribed range 

 of the few living descendents that the genus is approaching 

 extinction. 



THE GENUS PLATYCARYA 



The genus Platycarya was characterized by Siebold and Zucca- 

 rini, who have described so many oriental plants. It is a mono- 

 typic genus, that is to say, it contains a single existing species, 

 which was the basis of the genus Fortunaea of Lindley. This 

 single species is a small tree of Japan and northern China and 

 its range is roughly shown on the accompanying map (fig. 4) 

 in solid black. Monotypic genera usually have a \evy interesting 

 geological history, as for example Sassafras, Comptonia, Ginkgo, 

 and many others. However, no fossil remains of Playtcarya 

 have been discovered and this is probably due to the fact that 

 the vast continent of Asia is practically unexplored. 



