324 GEOLOGICAL SURVEY OF THE TERRITORIES. 



a family to which this genus belongs, are recognized in the same group. 

 From these considerations, I persist in regarding as ancient, primitive, or 

 derived representatives of a species of Liquidambar the fossil leaves de- 

 scribed under this generic name, until other specimens, if any are found, 

 mny point, by a variation of characters, to another more evident rela- 

 tion.* 



A number of vegetable remains of the Cretaceous are evidently 

 referable, by their characters, to Populus. The only dicotyledonous 

 leaves recognized by Heer, in the specimens wbich he studied trom the 

 Lower Cretaceous formations of Greenland, represent a Popuhis, appropri- 

 ately specified by the name of P. prwuBva. From a higher stage of the 

 same Cretaceous formation of that country, the celebrated Swiss pale- 

 ontologist has described three other species of Populus. In his Phyl- 

 lites Cretacees du Nebraska, and from specimens of the Dakota group, 

 he has recognized Populus lUigiosa, Populus f debcyana, and another 

 species still, P. cyclophylla, described in the Proceediogs of the Academy 

 of Natural Sciences of Philadelphia. Professor Newberry, in his paper 

 on the later extinct floras of North America, has described, also, besides 

 the doubtful P.f debeyana, three new species: Populus f cordifolia, P. 

 elliptica, and P. microphilla. The specification and the interrogative 

 puuctu?,tion applied to some of these names show that the authors them- 

 selves do not consider the generic reference as definitive, the character 

 of some of the leaves being somewhat in disaccord with those generally 

 recognized in species of Populus of our present time. Indeed, species 

 of this kind, like the present P. alba, for exami)le, have such multiplied 

 and diversified forms of leaves, such great variability in their nervation, 

 the mode of attachment of the petiole, &c., that they readily offer, by 

 comparison with fossil leaves of obscure relation, some points of afiQuity 

 which, being not found elsewhere, have to be consideretl by the authors. 

 Hence the doubtful references which may be, and are often rectified by 

 subsequent discoveries, as is proved by the great proportion of synonyms 

 appended to the enumeration of Populus species. To obviate this incon- 

 venient multiplication of fluctuating species of Populus^ I proposed a 

 new generic division, under the name of Populites, for the classification 

 of those Cretaceous leaves, numerous indeed, which, partaking of some 

 of the characters of Populus, are nevertheless removed frr na this division 

 by some others, as remarked in the memoir. t Populus lancastriensis 

 was considered as a legitimate species of the genus, and in the new 

 division were described Populiies elegans, P. ovata, P. quadrangiilaris, 

 P. flabellata, and P. sahsburicBfolia, with P. cyeiopliylla, represented by 

 leaves which I considered as answering to the description of this species 

 by Heer. 



This first memoir on some Cretaceous fossil plants from Nebraska 

 had to be prepared, at a short notice, from a limited number of speci- 

 mens. Since its publication, I have had opportunity to study the 

 specific forms of this Cretaceous flora by comparing a very large number 

 of specimens, and have thus recognized a more evident affinity of some 

 of those leaves referred to Populites with other generic divisions. The 

 only Popidites lancastriensis and P. elegans which Schimper considers as a 

 true Populus are preserved in this genus, while Populites cyclophylla and 

 P. ovata, appearing rather related by their characters to the Ampelidece, 

 are described under a new generic division. The leaves represented by 

 these species have, indeed, by their craspedodrome and subpalmate ner- 



* Fragments of leaves closely allied to this form are described as Phyllites in Reuss, 

 Verstein, PI. LI, figs. 4 and 5. 

 t Am. Jour. Sci., vol. xlvi, Jnly, 1868, p. 93. 



