DESCRIPTION OF SPECIES— SALIGINE^. 171' 



time, the North American continent lias tlie largest nnniber, nine, mostly 

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

 range of distribution: Populua tremuloides, Michx., and P. Canadensis, Desf., 

 lor example, extend between the Pacific and the Atlantic from Canada lo 

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

 to Mexico, descending as far soutii as Tampico. Anotlier, P. trichocarjia, 

 Torr. & Gray, is limited to California. Three species t»re 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 the fossil species, and leaving out (hose which have been 

 separated as doul)tful, we find described in Schimper's synopsis forty-two, of 

 which two only are Cretaceous: Populuslitigiosa, Heei-, and P. ekgans, Lesqx., 

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

 in the synopsis, P. cydofhijlla, Lesqx., and P. Lancasfriensis, Les(|x., from the 

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

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

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

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

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

 and the fourth, P. elliptica, of a Miocene type, seems 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 species of 

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

 the only dicotyledonous j)lant found in the flora of Comes, a Greenland Lower 

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

 dea, ConifercB, 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 tyjie already ])re- 

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

 flora is connjosed mostly of dicotyledonous plants. No speci(;s 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 rejire- 

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

 until now only two species, both from the lower members of the forniation, 

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

 iVuni the Fort Union and Yellowstone Lignitic, and wiiich may be Eocene, 



