Marsh Collection, Pedbody Museum. 425 



one region were gradually distributed to others by migration, 

 so that the deposits containing them would show the same or 

 closely allied species in practically the same stages of develop- 

 ment ; yet this similarity in the contained fossils may not of 

 necessity imply absolute synchronism of deposit, since a certain 

 length of time must have been required for the migration, 

 which would have been longer or shorter in proportion as the 

 distance was great or small and the obstacles to be overcome 

 were easy or difficult. It is just such complications as these 

 that arise when we compare the deposits of the Arctic with 

 those of the Temperate latitudes, and prevent a final and abso- 

 lute determination of their exact equivalency. 



In the North Temperate regions, the Mesozoic strata reach a 

 high degree of development and in many places contain rich 

 deposits of fossils. In Europe, the Triassic and Jurassic are 

 much better developed than in America, while on the other 

 hand the Cretaceous series is much more varied and extensive 

 in America than in Europe. 



The Jurassic of Europe has furnished a flora consisting of 

 Conifers, Cycads, Ferns, Equiseta, and two Monocotyledons. 

 In all, there are some one hundred and fifty species, of which 

 over eighty are Ferns, twenty are Conifers, and thirty or forty 

 are Cycads. The Conifers are represented by genera and 

 species closely allied to the Yew, Pine, Arbor- Yitse, Cypress, 

 and Norfolk Island Pine. The two Monocotyledons are said 

 to belong to the Arum and Pandanus families, or such types 

 in our living flora as the Calla, Skunk Cabbage, and the Screw 

 Pine. There are no Dicotyledons or higher Angiosperms 

 known with certainty. 



In America, the flora of the Jurassic is not so well repre- 

 sented, but the known species have a close relationship with 

 those of the Jurassic of Europe. Thus, Conifers, Cycads, and 

 Ferns are the only forms yet brought to light. The American 

 Jura has hitherto furnished no Monocotyledons, and as in 

 Europe, the Dicotyledons are completely absent. According 

 to all authorities, the climatic conditions indicated by this flora, 

 as well as by the associated animals, both Invertebrates and 

 Vertebrates, are such as are now found in the vicinity of the 

 Equator. 



The Lower Cretaceous of Europe is well developed and in 

 many places is capable of division into a number of substages. 

 In England, the deposits belonging in this series are the Weal- 

 den, Lower Greensand or Aptian, and the Gault or Albian. 

 In Portugal, at least six divisions are recognized, which corre- 

 spond almost exactly with the subdivisions of the Potomac 

 formation on the Atlantic border in this country. The flora 

 of the Wealden, according to Ward, contains eighty species, 



