DESCRIPTION OF SPECIES— SALIOINE^. ]71 



time, the North -American continent has the largest number, nine, mostly 

 inhabiting the cold and mountainous regions. Some, however, have a wide 

 range of distribution: Populus tremuloides, Mi(;hx., and P. Canadensis, Desf., 

 for example, extend between the Pacific and the Atlantic from Canada to 

 Louisiana and New Mexico. One species, P. Mexicana, Wesm., is peculiar 

 to Mexico, descending as far south as Tampico. Another, P. trichocarpa, 

 Torr. & Gray, is limited to California. Three species are common to Europe 

 and Asia ; even one of them, P. alba, L., has been found in Algeria. China 

 has two species, Japan one, and three belong to the Orient. 



Considering tlie fossil species, and leaving out those which have been 

 separated as doubtful, we find described in Schimper's synopsis forty-two, of 

 which two only are Cretaceous: Populus litigiosa, Heer, and P. elegans, Lesqx., 

 both from the Dakota group. To these we should add, though not mentioned 

 in the synopsis, P. cyclophylla, Lesqx., and P. Lancastriensis, Lesqx., from the 

 same formation, with P. hyperborea, Heer, P. Berggreni, Heer, from the Upper 

 Cretaceous of Greenland, and P. primceoa, Heer, from the Lower Cretaceous 

 of the same country. Dr. Newberry has described, in his notes on the later 

 extinct floras, four species, two of which are considered by himself as doubt- 

 fully referable to this genus ; another, P. microphylla, is of uncertain relation, 

 and the fourth, P. elliptica, of a Miocene type, seenjs to have been referred 

 to the Cretaceous by a mistake caused by misplacement of labels or of speci- 

 mens. Though it may be that we have already seven Cretaceous s])ecies of 

 Populus, one of which, P. primava, represents, by leaves and scales of seeds, 

 the only dicotyledonous plant found in the flora of Comes, a Greenland Lower 

 Cretaceous flora, composed of seventy-five species of Filices, Selaginece, Cyca- 

 dece, Conifera, and a few monocotyledonous, mostly of Jurassic or Wealden 

 types: this Populus is thus the oldest dicotyledonous plant known as yet. 

 From this, we cannot be surprised to find the generic type already pre- 

 ponderant at a higher stage of the Cretaceous, that of the Dakota group, whose 

 flora is composed mostly of dicotyledonous plants. No species of Populus, 

 however, has been described to this time from the Cretaceous of Europe. 

 This is a remarkable fact, rendered more striking by the scarcity of repre- 

 sentatives of the same genus in the Eocene of that continent, which has 

 mitil now only two species, both from the lower members of the formation, 

 Sezanne and Belleu. Besides five Tertiary species described by Dr. Newberry 

 from the Fort Union and Yellowstone Lignitic, and which may be Eocene, 



