WABU.] DISCUSSION OF THE TABLE. 517 



our present research is to inquire into the characteristics of the Laramie 

 flora as hitherto published and made known. 



The great profusion witli which the Dicotyledons are represented in 

 all these floras — amounting to considerably over half the species even 

 in the Senonian, over two-thirds in the Laramie, and nearly two-thirds 

 in the Eocene — makes this group of plants a somewhat more reliable 

 term of comparison than any of the less abundantly represented tyi)es 

 thus far considered. Whatever may be thouglit of the proper place of 

 the Gamopetala, so sparingly preserved for us in the fossil state, it is 

 universally admitted that the Apetala?,or Monochlamydea?, with their nu- 

 merous amentaceous genera, furnished the earliest representatives of 

 dicotyledonous vegetation, and that the forms with two floral envelopes 

 (Dichlaraydeae) came later and form a higher type of plants. If we ex- 

 amine the percentages here, we find that the law holds true for the Poly- 

 petalie and Gamopetaln?, which are the rising forms, or at least were 

 so during all three of the epochs under consideration. The percentage 

 is least in the Senonian, intermediate in the Laramie, and highest in the 

 Eocene. In the Ai)etaliB, however, the maximum development appears 

 in the Laramie instead of iii the Eocene, which is not easily explained 

 and probably will not continue to hold true with the more complete 

 elaboration of that flora. These comparisons are with the total floras 

 of the several groups, but perhaps a more interesting result will be ob- 

 tained if we consider the Dicotyledons by themselves, and then find the 

 relative proportions which the subdivisions bear to the whole in the 

 three formations. Such a comparison will show that in the Laramie 

 group the Apetalte. are 53, the Polypetahe 37, and the Gamopetahe 

 10 per cent, of the Dicotyledons ; that in the Senonian the Apetalse are 

 58.5, the Polypetaliie 33, and the Garaopetalae 8.5 per cent, of the Dico- 

 tyledons ; and that in the Eocene the ApetaliB are 42, the Polypetalte 

 46, and the Gamopetalas 12 per cent, of the Dicotyledons. On the theory 

 that these types progressed in the order named and that the Laramie 

 is intermediate between the other two formations, the relative number 

 of apetalous species should diminish as we pass from the Senonian to 

 the Eocene, which is the case, viz: Senonian, 58.5; Laramie, 53; Eocene, 

 42. The relative number of Polypetahe, on the other hand, should rise 

 with the age of the strata, and this we also find to be the case : Senonian, 

 33; Laramie, 37 ; Eocene, 46. The Gamopetalje should also rise with the 

 strata, but more rapidly. The figures are : Senonian, 8.5; Laramie, 10; 

 Eocene, 12. These coincidences of fact with theory are interesting, and 

 in view of the circumstance that they continue to hold from the Ceno- 

 mauian below to the Miocene above,' they can scarcely be regarded as 

 wholly without significance. 



The advantage of comparing such large classes consists in the tend- 



' See Fifth Annual Report of the United States Geological Survey, 1883-'84, pp. 449, 

 450. For similar data for the comparison of the floras of other formations, see table 

 on pages 440 and 441. ■ 



