190 STANTON—MOLLUSCAN FAUNULE. [April 3, 
large area, so that their exact stratigraphic relations with each other 
are not observable. It will therefore be necessary to consider the 
different formations and horizons that have been recognized, in 
order to make an approximate determination of the age of the fossils 
now under consideration. 
In southern Wyoming the marine Jurassic is immediately over- 
lain by the Como beds (formerly called Atlantosaurus beds), con- 
taining a large reptilian fauna and a considerable number of Unios 
and other fresh-water shells. Similar beds that are correlated with 
them by means of the fossils occur in the Black Hills, along the 
Front Range in Colorado and elsewhere in the Rocky Mountain 
region. These beds have usually been referred to the jurassic, 
though recently several paleontologists have referred them to the 
Lower Cretaceous. The mollusca are of modern types, mostly 
belonging to genera that are still represented by living species, but 
the specific forms are quite distinct from all those found fossil in 
later beds. Of all the species that have been assigned to the Como 
horizon only one (Viviparus gilli M. and H.) is comparable with 
a form in this Montana collection, and that one is from a locality 
near the head of Wind River, Wyoming, where it was associated with 
Liaplacodes veternus M. and H. and Weritina nebrascensis M. and 
H. As none of these three species has been found elsewhere,’ the 
age of the bed from which they came is doubtful, and may well be 
later than the Como. 
Two non-marine formations of this general region, the Cascade 
and the Lakota,” have been referred to the Lower Cretaceous. The 
Cascade formation, which occur in the neighborhood of Great 
Falls, Montana, is coal-bearing and has been correlated by means 
of the fossil plants with the Kootanie of the neighboring Rocky 
Mountain region in Canada, and with the lower Potomac of the 
Atlantic border. Its geographic position and the apparently simi- 
ilar stratigraphic relations favor the supposition that the fresh-water 
beds near Harlowton may belong to the Cascade (Kootanie) for- 
mation, but unfortunately the former have yielded no plants except 
fossil wood, and the fauna of the latter is practically unknown. 
Obscure imprints cf Unio have been reported from the Cascade, 
but nothing sufficiently definite for description. A few undescribed 
1 Logan inadvertently describes Planorbis veternus as Liaplacodes from the 
Freeze-out Hills of Wyoming, in Kansas Univ. Quart., Vol. IX, p. 132, 1900. 
2 Named by Weed in the Fort Benton Folio, Geo/. Atlas of the United States. 
