REVIEWS 775 
about too species, most of them either Jurassic survivors or of Jurassic 
aspect. About forty of the species do not survive the relatively short 
hiatus following the Patuxent. 
The Arundel formation consisting typically of drab, more or less 
lignitic clays, carrying iron carbonate or siderite in segregations of 
varying forms, attains a maximum thickness of perhaps 125 feet in the 
middle of the belt in central Maryland, whence it thins to the seaward, 
with a general dip to the southeast of about 50 feet to the mile. This 
formation has not furnished a rich flora, but it is remarkable as the 
source of all the saurian vertebrates, including eight dinosaurs and one 
crocodile, discovered in the Potomac group. It has furnished also the 
three fresh-water gastropods and one of the two fresh-water pelecypods. 
The Arundel flora, numbering but 33 species, four only of which are 
not known in either of the contiguous formations, is botanically much 
more closely bound to the underlying Patuxent than to the Patapsco. 
The vertebrates of the Arundel point toward Morrison age which Pro- 
fessor Lull, in agreement with Williston and many other geologists, is 
disposed to regard as Cretaceous, at least in part. The flora is appar- 
ently bound to that of the Kootenai, also Cretaceous. According to 
Berry the base of the Kootenai, so far as the latter is known by its 
fossil plants, is slightly older than the base of the Patuxent. Berry 
and Lull accordingly agree that the Patuxent and Arundel formations 
are of Neocomian and Barremian age. 
A considerable hiatus (Aptian) appears to have elapsed before the 
Patapsco formation, embracing over 200 feet of sands and clays, notably 
variegated argillaceous material, covered and even considerably trans- 
gressed the areas of the Patapsco and Arundel. Of fossil animals, a 
single species, Unio patapscoensis, has been found in this formation. 
However, the Patapsco flora is both interesting and important. The 
contrast of the flora, embracing 83 species, is marked not only by 
extinction of the earlier plants (including 17 ferns, 24 cycadophytes, 
1 ginkgo, 12 other gymnosperms, and all of the 7 supposed primitive 
angiosperms) but also by the introduction of higher forms of distinctly 
younger aspect. For in addition to the 41 Patuxent-Arundel species 
which survived from the older formations there are found 42 new types, 
including many unquestionable angiosperms. It appears that during 
the interval between Arundel and Patapsco time the dicotyledons which 
were destined so soon greatly to outnumber the plants of all other 
classes in the upper Cretaceous floras had already made a good start. 
The limit of space forbids even mention of the wealth of philosophical 
