582 TIMOTHY WILLIAM STANTON 
attributed in part to the influence of climate, but it would carry 
us too far from our subject to discuss this question fully. 
The life of the Cretaceous period offers many other points of 
general interest, chief among which is the fact that while it is 
essentially Mesozoic in character, and is thus allied with the life 
of earlier periods, it nevertheless includes the earliest recorded 
repres ntatives of very many recent generic and family types 
and some groups of higher rank, and in the later stages this 
modern element is often relatively large. This statement refers 
chiefly to plants and invertebrates, for, with the exception of 
the Teleost fishes, which are introduced for the first time, the 
vertebrates nearly all belong to types now extinct or of rela- 
tively little importance. So far as the record goes the great 
group of placental mammals was not yet introduced, and the 
vertebrate fauna consists largely of Mesozoic types of reptiles, 
such as dinosaurs, pterosaurs and pythonomorphs, with a few 
small mammals of the lowest groups, and birds of archaic types. 
Among the invertebrates the Ammonoidea are very greatly 
differentiated and finally become extinct with the close of the 
period. Many other forms, such as Inoceramus, certain types 
of Ostreidae, Nerinea, Anchura, Pugnellus, etc., do not pass the 
upper limits of the Cretaceous. On the other hand, the Ostre- 
idae, Anomiidae, Mytilidae, Unionidae, Veneridae, Mactridae, 
Turritellidae, Naticidae, Volutidae, etc., are represented by 
forms closely related to living species. The flora is completely 
revolutionized and modernized during the Cretaceous. Early in 
the Cretaceous the first Dicotyledons occur and by mid-Creta- 
ceous time they largely predominate and numerous genera of 
trees that time still live in our forests are already introduced, so 
that the biologist who is studying recent species must often go 
back to the Cretaceous faunas and floras to complete his data 
for a rational classification. 
RECOGNITION OF THE LOWER CRETACEOUS IN THE UNITED STATES. 
Before going farther it may be well to recall the general clas- 
sification of Cretaceous deposits as adopted in Europe. It is 
