THE WALNUTS AND HICKORIES 71 



The existing species of hickories number from 8 to 15, 

 according to the rank assigned to the varieties of the 8 or 9 

 easily distinguished main types. They fall naturally into two 

 groups — the true hickories, and the pecan hickories — groups which 

 were already clearly defined in preglacial Pliocene times. 



The true hickories are fine, slow growing trees in general tem- 

 perate dry soils, and with hard strong wood. The buds are full 

 with overlapping scales, the nuts are generally thick shelled and 

 thick husked, and each leaf consists of from three to nine leaflets. 

 The pecan hickories are trees which require warmth and moisture, 

 and have relatively weak wood. The buds are thin and narrow 

 without overlapping scales, the nuts have thin shells and thin 

 husks, and the leaflets are numerous, slender and falcate. 



Over a score of ancestral fossil species are known. Unlike the 

 walnut, the hickory is not certainly known from the Cretaceous 

 period, but it is present in nearly every early Eocene deposit in 

 Wyoming and on our Pacific coast. Hickories occur in the upper 

 Eocene of Central Europe and our Gulf States, and there is a fine 

 large leafed form in deposits of this age at Kukak Bay, Alaska. 

 A form known as Ilicoria ventricosa is very abundant in the brown 

 coal deposits of Europe of Oligocene age. The late Miocene appears 

 to have been the period of widest extent of these trees, representa- 

 tives having been found throughout Europe and North America in 

 regions where they no longer exist, as on the former continent, or 

 in Oregon, Cahfornia and Colorado. 



During the succeeding Pliocene period the hickories are as abun- 

 dant and vigorous as in the late Miocene in Europe although their 

 northern limit appears to have become somewhat restricted. Even 

 as late in geologic time as the upper Pliocene several species were 

 abundant in Italy and Germany, but none survived the Ice Age 

 on that continent. A form resembling the modern pecan is repre- 

 sented by both leaves and nuts in a late Phocene coastal lagoon 

 deposit in southern Alabama. 



We have, in America, numerous Pleistocene records, the leaves 

 being preserved in the clay deposits of the river terraces, and the 



