ad PROCEEDINGS OF THE NATIONAL MUSEUM. 489 
that higher types of plants were reached in these strata than in any 
synchronous with them in Europe. 
Returning to the consideration of the constituent groups of plants in 
| the Great Falls flora, we find the ferns decidedly dominant, but differ- 
ent species are by no means equally represented. At the head of the 
list in number of specimens stands Aspidiwm montanense, a new species. 
Next to this, and in about the order of naming, come Aspidium mono- 
-carpum, also a new species, Thyrsopteris rarinervis Font., and Osmunda 
dicksonioides Font. These were all apparently abundant. To judge 
_ from the character of the rock which bears the imprints, these did not 
~ all occur on the same horizon. Osmunda dicksonioides, for example, 
- occurs in a material very different from that which shows the imprints 
| of Aspidium montanense. 
In this connection I will repeat an opinion expressed before. In 
determining the age of an unknown group of fossil plants, greater 
- weight as evidence of age ought to be assigned to some plants than 
_ to others. These are the plants whose fossils have marked and salient 
features that permit them to be identified without danger of error. An 
example of this kind of plant is Frenelopsis, especially /. parceramosa, 
| _ of the Potomac flora. When these are fully established and at home 
| in a formation, as would be shown by their general distribution and 
the abundance of the fossil specimens that they afford, they ought not 
to be counted simply as units in a sum total to establish a percentage. 
1 Their evidence would thus be neutralized by that of other units which 
| are newcomers or belated survivors. This is especially true of floras 
in a critical stage of evolution, and which contain considerable numbers 
of newcomers and survivors. The Potomac flora was one of this char- 
acter, in which Jurassic types were being cast out and Cretaceous ones 
; introduced. If the era of deposition of the Great Falls beds was nearly 
| that of the Potomac, as is most probable, then the flora of the time 
_ must have been likewise in an unstable condition. 
| So far as yet made out, the ferns seem to be the most common plants 
in the Great Falls flora. The above remarks, owing to the character 
of this type of vegetation, and to its long persistence with but little 
- change, do not apply to them so well as to many of the forms found in 
the Potomac flora. Thyrsopteris rarinervis, however, is a fern that has 
a well-marked facies, which is not possessed by many ferns. Its abun- 
dance in the Great Falls strata gives it great weight in establishing a 
resemblance between the Potomac and Great Falls floras. 
Equiseta appear to have been very rare in the flora now in question. 
The few imprints that are found are very poorly preserved, and seem 
to have been made by fragments that had floated a long time in water. 
The two collections yield the following species: 
Equisetum Lyelli? Mantell. 
. 
Only one imprint was seen that was clearly that of an Equisetum. 
It occurs in a ferruginous, sandy shale. It is too poorly preserved to 
