7" Later Extinct Floras of North America^ 



In texture the leaf was apparently similar to thai of /'. oeci- 

 dentalis. rather thin and more or lesB roughened. 



Formation and Locality. Miocene Tertiary deposits. Banks 

 of Yellowstone River. (Dr. Hayden.) 



Plat mm* Ilayrieiiii. (n. sp.) 



Leaves large, long-petioled, when mature three, perhaps rarely 

 five lobed : lobes nearly equal, long-pointed, acute ; on either side of 

 the middle lobe five to eight obtuse teeth; margins of the lateral 

 lobes sinuately toothed to near the base; younger leaves ovate, 

 acuminate, coarsely toothed throughout excepl near the base, which 

 ie slightly decurrent; nervation strong, radiate from the base, 

 primary nerves three, which are nearly straight and terminate in 

 the three lobes of the border. From the midrib spring seven or 

 eight pairs of lateral nerves above the basilar pair; these diverge 

 at an angle of about 35°, are slightly flexed at the base, straight or 

 nearly so above, where they are somewhat truncated, their branches 

 terminating in the marginal teeth. The basilar nerves diverge 

 from the midrib at an angle of about 35° and run nearly straight 

 to the extremities of the lateral lobes. They each give oft' on the 

 lower side seven or eight branches, of which the second or third is 

 strongest. These are more or less curved and branched, the 

 branches terminating in the teeth of the margin. Fruit 2— 8 lines 

 long, prismatic, clavate. 



This tine species, which is well represented in the collection, 



is closely related to Platanus aceroides^ so common in the 



Miocene strata of Europe. There are, however, noticeable dif- 



nces which seem to rae to have a specific value. The leaves 



of /'. aceroides, though exhibiting great variety of form, are 



1 belieVe alway> acutely toothed, while in the s] x-citnelis before 

 118, the teeth are never acute except those which in the young 



leaves represent the lateral lobes of the mature form. In /'. 

 aceroides also, according to Heer (Flor. Tert. Helvet. v. ■"• 

 j». 71. Taf. Ixxxvii. and Ixxxviii.), the nervation is more 

 sparse, the angle of divergence <>f all the nerves greater, the 

 number of lateral branches of the midrib less, and the number of 

 marginal teeth considerably greater. Prof. 1 1 em- says (loc. cit.) 

 thai in /'. aceroides the middle lobe of the leaf lias 2 1 den- 



