DESCRIPTIONS OF SPECIES. 39 



nervation radiate; medial nerve straight, simple below, branched near the 

 summit; lateral nerves, three pairs diverging at nearly equal angles, from a 

 common point of origin; lower lateral nerves small, simple, arched upward 

 at their summits, terminating in the margins ; second pair of lateral nerves 

 springing from the basal point of radiation nearly at right angles with the 

 midrib, arching upward as they approach the lateral margins, and support- 

 ing each about three branches on the inner side; third pair of lateral nerves 

 diverging from the midrib at its base at an angle of about 45 degrees, 

 bearing one or two lateral branches, and terminating in the margin above 

 the middle of the leaf." 



Of this neat species there are no complete specimens in the collection 

 made by Dr. Hayden, none of them showing the summit of the leaf. Enough 

 is, however, discernible in them to show that they represent a species 

 of Populus different from any other in the collection and from any 

 before described. Of the species at present growing on the North Ameri- 

 can continent the leaves of P. heterophylla approach most nearly to 

 these, but the nervation of the leaves of that tree is never so distinctly 

 radiate. 



In the character of its marginal dentations this species resembles P. 

 mutabilis crenata Heer, but is clearly distinguished from that by its cordate 

 base and corresponding radiate venation. 



Populus Zaddachi Heer (Fl. Tert. Helv., Vol. Ill, p. 307) has a still 

 closer resemblance to this than either of the species mentioned, and it has 

 been regarded by Lesquereux as identical with it, but in all the figures 

 of that species published the dentation of the margin is less strong and acute 

 and the nervation is less radiate. 



In P. cordata the basilar pair of lateral nerves reaches the margins below 

 the middle of the leaf, and the second pair of lateral nerves spring from 

 nearly the same point, while in P. Zaddachi the basilar pair reach the margin 

 above the middle and the second pair leave the midrib considerably above 

 the origin of the basilars. 



The leaf figured by Professor Heer (Fl. Foss. Alaskana; Fl. Foss. Arct, 

 Vol. II, Abth. II, PI. II, fig. 5), has the character of the fossil before us and 

 would seem to represent the same species. Yet notwithstanding the differ- 

 ences already pointed out, this is referred by Professor Heer to P. Zaddachi. 

 The nervation is, however, so different from that of the typical forms of that 



