DESCRIPTION OF SPECIES— BETULACE^. * 137 



of specimens, mostly fragments of leaves, which represent a species inter- 

 mediate, by its characters, the form, and the size, between this and the former. 

 The nervation is of the same type. These fragments show such an intimate 

 relation between M. Lessigii and M. insignis that both appear necessarily 

 referable to the same generic division. 



Habitat. — Coal Creek, Colorado, in clay overlying coal, reached by a 

 shaft sixty feet deep [Gen. W. H. Lessig). 



BETULACEJ]. 



BETULA, Linn. 



The distribution of this genus is limited at our epoch to the northern 

 regions of Asia, Europe, and America, a few of its species ascending to the 

 Arctic zone. Of the twenty-nine species described in the Prodromus of De 

 Candolle, eight inhabit North America, four of them exclusively belonging 

 to its flora. The numerous species of Betula described from the Tertiary of 

 Europe, thirty-nine, are especially related to the present North American 

 forms, as are also the few recognized in our geological formations. 



The generic type appears to have originated in the Cretaceous period; 

 for we have already two species described from the Dakota group formation: 

 Betulites denticulata, Heer; Betula heatriciana^ Lesqx. The genus is repre- 

 sented also in the Eocene of Europe by three species, two of them in the flora 

 of Sezane; by four in the Paleocene, and thirty in the Miocene. Of these, of 

 course, a large number are uncertain, the specific determinations from leaves 

 only being perhaps more unreliable for this genus than for any other. In 

 this country, one leaf only has been found in the lignite of Golden, doubtfully 

 referable to Betula gracilis, Ludw., which by itself is already of uncertain 

 relation, the only leaf which represents it being related to Populus rather than 

 to Betula. Therefore we do not have as yet any positive record of this genus 

 in the North American Lower Eocene. It is present, however, at Evanston, 

 or in Upper Eocene, by two species, one of them new; also at Fort Fetter- 

 man, a Miocene formation, where some leaves of a new species have been 

 found in connection with a profusion of remains of Taxodium iniocenicum. 



Betula Vogdesii, Lesqz. 



Plate XVII, Figs. 18, 19. 

 Betula Vogdesii, Lesqx., Annual Report, 1874, p. 312. 



Leaves small, thin, oval, acutely pointed, narrowed, and rounded to the petiole, minutely serru- 

 late, penuiuerve ; lateral veins parallel, opposite at or near the base, simple or the lowest ones sparingly 

 branching, craspedndroujo. 



These leaves vary in size from three to four centimeters long and from 



