ANALYSIS OF TUE J)AKOTA GROUP FLORA. 253 



Viv'ing Friiiif/iiln ((trolii/idiia Gcvny; while R. inceqnilufrralis, iihn oi' t]iv. Dakota 

 Group, is typically allied to R. (enuujcnsls of the Miocene ot" Europe. ()t 

 the Rhauiuea', Geliudeu has only one species, a Zizyphus. Hut the ditiereut 

 genei'a of the family become more and more richly represented in tlu; Ijar- 

 amie Group and the more recent formations. From the Laramii^ flora l*rof 

 Ward, besides Bcrcheuiid niidfiiierriti, a species common in tlie Tertiary of 

 both continents, has recognized three species of Zizvi)]nis, oueof tliem new; 

 three of Paliurus, one of which, the most conunon, is 1'. ('oloiiihl lleer. 

 From difterent localities refeired now to the ].iaraniie, 1 liave recorded ten 

 species of Rhamuus and one of Ziz}j)hus, l)esides two s})ecies of Paliurus, one 

 of Zizyphus, two of Rhaimius from the Green River Group, and one of Zizy- 

 phus and two of Rhanmus from the Upper j\riocene of the auriferous g-ravel 

 deposits of California. A line species of Rhamnus is also described in the 

 l\Iississippi flora (Eocene). The types of all these genera represented in 

 the Dakota Grouj) may be followed by their atliliation and clearly recog- 

 nized through the ge^ilogic ages to the i)resent epoch, where the flora of 

 North America has still in the Atlantic States one species of Rerchemia, 

 three of Rhamnus, and four of Ceanothus, while it has on the Paciflc slope 

 four species of Rluunnus, eighteen of Ceanothus, and one of Zizyphus. 

 Tlie preponderance of species of Ceanothus in this last flora is remarkable, 

 and does not appear to result from ancestral influence, for no other repre- 

 sentative of this genus has been observed in the Tertiary of North America, 

 except Ceanothus 3fcifjsil Lesq., of the Mississippi Eocene, a species dis- 

 tinctly related to the living C. Anivricaitus Linn. 



Of the order Juglandea?, Juglans is as yet the only genus of which 

 leaves have been observed in the Dakota Group. One species, Jiif/laiis 

 ardica Heer, first described from the schists of Atane, has been later recog- 

 nized in the Dakota Group with ./. prmonUalis Lesq. and two other forms 

 of leaves, which have been described under the generic name of Juglandites 

 on account of their insufficiently ascertained relations. Jii/jlans crassipc'^ 

 Heer, of the Cenomanian of Moletein, is also recognized in the Seuonian of 

 Patoot, and Da^\'son has described -/. harwoodevsls from the Upper Creta- 

 ceous of Vancou^•er Island, and has recognized J. cretacea from the Peace 

 and Pine River series of Canada. In more recent geological times the genus 

 becomes more abundantly represented. The Laramie Group has seven 

 species of Juglans and one Carya ; the Eocene of the Mississippi two species 

 of Juglans; the Green River Group has five species of Juglans, with one 

 species common also to the Laramie Group, and four opecies of Carya. And 



