518 FLORA OF THE LARAMIE GROUP. 



eiicy of this method to eliminate the disturbing element of geographical 

 distribution, which, as we shall soon see, is the chief obstacle to exact 

 results in the consideration of genera and species. The species may all 

 differ, tiie genera may be more or less local, even the orders may pre- 

 vail in certain continents or hemispheres, but the relative predominance 

 of such great types as the vascular Cryptogams, the Gymnosperms, or 

 the Dicotyledons may depend chietiy upon the period in the historj' of 

 their development, and, therefore, afford a measuie of time which is as 

 much more reliable as it is more rude and general than that aftbixled by 

 the narrower groups of vegetation. Viewed in this light, the data thus 

 far considered, while suggesting nothing more definite, may be fairly 

 claimed to prove that the Laramie age was considerably later than that 

 of the Senonian, and somewhat earlier than that of the Eocene flora. 



In the classification of plants according to the natural method the 

 next subdivision after the ones we have just considered is that into 

 natural families or Orders. In certain large systematic works, it is true, 

 an intermediate group is often introduced, usually called the Cohort, 

 but it will not be necessary or convenient in the present case to treat 

 this subdivision separate from the Order. In the cellular Cryptogams 

 the classification is very unsettled, and the several groups receive dif- 

 ferent systematic values. The Fungi, Lichens, and Algie are not always 

 regarded as orders, but they are so rare in a fossil state and of so small 

 importance from the chronological point of view that they may be con- 

 veniently so regarded here. Four sjjccies of Fungi, consisting chiefly 

 of spots on dicotyledonous leaves {Sphwria, Hall) have been described 

 from Laramie strata, while only one such has been reported from the 

 Senonian and only two from the Eocene. The only lichen referred to 

 any of these formations is an Opegrapha from the Laramie. Nearly a 

 hundred species of supposed AlgiC have been published from the three 

 horizons, three-fourths of which are Eocene, embracing a large number 

 of doubtful forms described (and often not figured) by Massalongo from 

 Monte Bolca, etc. The Laramie furnishes only eight and the Senonian 

 seventeen. Their diagnostic value may be set down as nil. Twelve 

 Characete (all belonging to Chara) and four Muscineje all from the 

 Eocene, complete the cellular Ci-yptogams, which, for our i>resent pur- 

 pose, might as well have been omitted from the table. 



The Filices, or Fern familj^, constitute an important order from the 

 point of view of this discussion, furnishing 154 species. As the waning 

 descendants of Carboniferous types that predominated throughout the 

 earlier history of the globe, we naturally expect them to continue to 

 bear in point of abundance some relation to the age in which they are 

 found, the earlier to have precedence over the later. The assumed 

 position of the Laramie group between the other two is borne out by 

 this order, for, although a larger actual number of species occurs in the 

 Eocene than in the Laramie, this number is less in proportion to the 



