LOWER CRETACEOUS FORMATIONS AND FAUNAS 589 
with other independent sources of evidence, for the Portuguese 
plant beds are interstratified with strata carrying marine faunas 
that are, as we shall see, closely related to the fauna of the Com- 
anche series, and one of the lower horizons in the Comanche has 
yielded fossil plants closely connected with the flora of the lower 
Potomac. 
Invertebrate fossils are remarkably scarce in the Potomac, 
and the few that have been found do not afford any definite 
evidence as to the age of the beds. The only mollusks known 
from the lower horizons developed in Maryland and Virginia, 
are a few internal casts apparently belonging to small simple 
forms of Unio, whose only geological value is to show the fresh- 
water origin of the beds. In New Jersey, besides some Unios 
that probably came from a much later formation, five species of 
mollusks have been reported from the Raritan formation.* They 
have been referred to Astarte, Corbicula, Gnathodon, and 
Ambonicardia (gen. nov.), but not one of them is well enough 
preserved to show generic characters, and in invertebrate paleon- 
tology, at least, age determinations, based on new species of 
doubtful genera, are worthless. 
Vertebrate fossils have been collected from the Lower Poto- 
mac in a limited area between Washington and Baltimore. One 
species based on a tooth, Astrodon Johnsoni, has been described 
by Dr. Leidy, and Professor Marsh? has named five others: 
Pleurocoelus nanus, P. altus, Priconodon crassus, Allosaurus medius, 
and Coelurus medius. He states that associated with these there 
are remains of crocodiles and_ tortoises, of Jurassic types, 
some fishes and a few mollusks. Also that ‘‘The fossils here 
described, and others from the same horizon seem to prove con- 
clusively that the Potomac formation in its typical localities in 
Maryland is of Jurassic age and lacustrine origin.’”” The genera 
Allosaurus and Coelurus were originally described from the 
Atlantosaurus beds (Jurassic) of the Rocky Mountain region, 
and Pleurocoelus has since been found in the same beds repre- 
*Monograph 9, U.S. Geol. Surv., pp. 22-28, 
? Am. Jour. Sci., 3d. ser. Vol. XXXV, 1888, pp. 89-94. 
