Maryland Geological Survey 147 



Lower Cretaceous as a whole. While this necessarily represents but a 

 small percentage of the species which clothed the earth during that period, 

 it furnishes some data bearing on the march of vegetation during which 

 the transformation from a Jurassic to an Upper Cretaceous and essen- 

 tially Cenozoic type occurred. This flora shows evidence in the varying 

 proportions which the main types, such as the ferns, cycadophytes, and 

 conifers, bear to one another, that we have represented plants which grew 

 under considerable local differences of soil, altitude, humidity, and pre- 

 cipitation conditions. 



It is apparent that the dominant types of the late Jurassic floras con- 

 tinued without marked change throughout the older Cretaceous. These 

 are the ferns, cycadophytes, and gymnosperms. We know little about 

 the Thallophyta, the Bryophyta, or the Lycopodiales. The Equisetales 

 had evidently dwindled to proportions strictly comparable to their present 

 day deployment. The more characteristic fern families of the older 

 Mesozoic, such as the Marattiacese, are greatly reduced in importance, 

 and the families ^ Schizseacese, Gleicheniacege, Matoniacea), Osmundacese, 

 and Dipteriacese, which are of great importance in the early part of the 

 Lower Cretaceous, were destined to be overshadowed by the Polypodiacese 

 before the close of the Cretaceous, ferns of this type represented in the 

 eastern United States by various species of Cladophlehis and Onychiopsis 

 being already the most abundant numerically even as early as the begin- 

 ning of the Patuxent. Pteridospermge are unknown, and it is within 

 reason to suppose that this class was no longer represented in the flora 

 of the world. 



The Cycadophytes of the early Cretaceous are essentially the familiar, 

 even if too little known, types of the later Jurassic. They are abundant 

 in genera, species, and individuals, and are quite as dominant an element 

 in the earlier Cretaceous as in the Ehsetic and Jurassic floras. Before 

 the close of the Lower Cretaceous, however, they became largely extinct. 

 The other gymnospermic types — the Ginkgoacese, Taxacecs, and Pinacese 



^Ttese family names are used in a generalized sense and not as if they 

 were coterminous with our modern groups of the same names. 



