PALEONTOLOGIC HISTORY OF PLATANUS. 



several are found in the European Miocene. The Laramie group of the 

 Kooky Mountain region, a formation which occupies a disputed position 

 between the Cretaceous and Tertiary, and seems to span the boundary 

 of Mesozoic and Cenozoic time, furnishes about half the known fossil 

 forms. The species from this formation are all founded on the impres- 

 sions of leaves, no inflorescence nor fruit having thus far been discov- 

 ered. Among these leaves are some that deviate widely from those of! 

 living plane trees and seem to resemble those of Aralia. The most re-' 

 markable of these is the noble plane (Platanus nobilis) of Newberry, from: 

 the Upper Missouri country, or Fort Union group. This tree had a very 

 large leaf, sometimes with a breadth of 2 feet, bearing a number of lobes,.. 

 palmately disposed, and a considerable portion of the margin of the leal 

 was destitute of indentations or teeth. Smaller leaves having essen- 

 tially the same form and nervation, but usually somewhat smoother om 

 the margins, have been referred to Aralia. I have, however, collected 

 gr at numbers of these leaves from beds on the Lower Yellowstone, 

 where all the intermediate forms and sizes occurred in immediate asso-> 

 eiation, so as to leave no doubt in my mind that they all belonged to the' 

 same type of plants. Fig. 1, PL XVII, represents one of these smaller: 

 forms, natural size. 



This leaf has the usual form at the base for both the large and the 

 small specimens, but ethers occurred having a remarkable expansion at 

 the base of the blade, projecting backward on the leaf stalk and having 

 from 2 to 5 lobes or points, as shown in figs. 2-5, Pis. xvii-xix. 



These expansions are to be interpreted as evidence that the leaves: 

 all belong to Platanus or to some extinct ancestral type of that genus, 

 since something quite analogous to them is found in our American 

 plane tree. The ordinary leaves of this tree are, it is true, destitute of' 

 basilar expansions, but those on young shoots, and sometimes those om 

 the lower or non-fruit-bearing branches of trees, exhibit this peculiarity. 

 Fig. G, PL xix, which represents a leaf from a small tree, shows it with 

 considerable distinctness. Though less prominent, its resemblance tO' 

 that of the fossil leaves is quite close. 



In place of this backward expansion of the blade many sycamore' 

 leaves have an appendage similar in shape at the base of the leat stalk, 

 as though the once basilar appendage had been separated from the 

 blade and crowded down the petiole to its point of insertion. This is 

 very clearly shown in fig. 7, PL xx. from a young shoot with wedge- 

 shaped leaves and very short petioles. More frequently these miniature 

 blades are forced entirely oft the petiole and are found grown together' 

 around the stem above the attachment of the leaf, so as not even to 

 constitute true stipules. The constriction seen in the fossil forms be< 

 tween the blade of the leaf and the appendage would seem to represent 

 the beginning of this process of detachment of the latter, and there is 

 another fossil form {Platanus appendiculata Lx., fig. 8, PI. xx) found in 

 the much more recent auriferous gravels of California, which corre- 

 >; uds precisely in this respect with the living specimen last figured. 



