152 ORTOIKAL AETTCLES. 



The character of veuation ofteu differs very materially in the 

 same natural genus. Take, for instance, Liquidambar, one of the 

 older generic types of Dicotyledons, and compare L, styraciflua, 

 L. cliinense, and L. Altingia, both in respect of form and venation 

 of the leaves. The nervation and venation in Loranthus is very 

 variable ; also in South American species of Coiissapoa. Compare 

 G. calophylla, PL, C. fontanesiana, Tree, (C. sylvatica, PI.)? ^^^ 

 G. trinervia, Spr. Compare also the species of Styrax, and such a 

 list might be indefinitely extended. 



I have not, in tabulating, restricted myself to fossil genera, the 

 determination of which has been based upon indubitable evidence ; 

 though by attaching, in the table, p. 175, a numerical reference to 

 several of the genera which appear doubtful, and which are remarked 

 upon at the end of this pajjer, I have partly distinguished between 

 those which, to the best of my judgment are to be depended upon, 

 and those which should be accepted with more or less doubt. I say 

 partly distinguished, for I have thus marked only about thirty genera, 

 though I believe that .fully one hundred of these generic determina- 

 tions are more or less doubtful. 



In the notices which follow, all reference to Cryptogamous plants 

 is omitted ; partly, because with the exception of the vascular groups, 

 the fossil data are almost valueless ; partly, because I have myself 

 but a very limited acquaintance with the most important of the vas- 

 cular orders — Filices, and partly, because I believe they do not 

 afford material additional evidence aftecting the principal question 

 discussed. I have introduced several statistical items of informa- 

 tion which do not directly bear upon the dispersion of the tertiary 

 flora and the hypotheses of Atlantic or Pacific migration, but these 

 have appeared sufficiently interesting on independent grounds. I 

 cannot claim for the numerical details anything like absolute accu- 

 racy, though I believe them to be trustworthy in the main. 



The Tertiary Flora of Europe ; its general character, ^c. This 

 is admirably reviewed by Professor Heer in his essay on the climate 

 and vegetation of the tertiary epoch. As this is separately published 

 at a very moderate price I must refer to it for detailed information 

 and confine myself here to principal features. The Swiss tertiary 

 remains of Phgenogamous plants, exclusive of " incertae sedis," are 

 distributed through 80 natural orders, and about 196 genera, 

 (Dieots. 160, Monocots. 36), of which 151 (Dicots. 133, Monocots. 21) 

 are yet existing types. The total number of species of Phanerogamia 

 is estimated at about 800, of wliich number nearly half are referred 

 to nine or ten natural orders. These latter are as follows: — 



species 25 

 „ 23 

 „ 21 

 „ 20 



The sequence of the above largest orders varies if the four stages 

 of the tertiary deposits be separately considered. In tlie first and 



