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are not transitional between the Mesozoic and Tertiary forms, but their 
affinities are with the former beyond a doubt; thus indicating a great 
faunal break. * * * and the great break is between this horizon 
[the Peurco] and the Ceratops beds of the Laramie. * * * It is safe 
to say that the faunal break as now known between the Laramie and the 
lower Wasatch [Puerco] is far more profound than would be the case if 
the entire Jurassic and the Cretaceous below the Laramie were wanting. 
Cope (Amer. Naturalist, xxvi, 1892, p. 762), quoting from Marsh the 
words ‘“‘the more the two [Laramie and Puerco] are compared the stronger 
the contrast between’, adds: 
It is true that no Ungulata have yet been found in the Laramie, while 
they abound in the Puerco, but we cannot be sure that they will not yet be 
found; the probabilities are that they existed during the Laramie and 
that it is due to accident that they have not been obtained. But the Multi- 
tuberculata of the two faunz are much alike. 
Osborn (Bull. Amer. Mus. Nat. Hist., v., 1893, p. 311) writes: 
This Laramie fauna is widely separated from the Upper Jurassic, and 
is more nearly parallel with the basal Eocene forms of the Puerco and the 
Cernaysian of France. * * * These conclusions are directly the re- 
verse of those expressed by Marsh in his three papers upon this fauna. 
Cross (Geology of the Denver Basin, p. 220) concludes that this differ- 
ence of opinion deprives the mammalian remains of much of their value 
in the present discussion. 
To the present writer Marsh’s opinion seems to be erroneous. Geo- 
logically, of course, the Jurassic mammals are much farther removed 
from those of the Lance Creek beds than the latter are from those of the 
Puerco, Torrejon, and Fort Union. The same remark may justly be made 
regarding the stage of development attained by the Jurassic mammals. 
Systematically considered, the case is different; and the solution of the 
problem depends on the systematic relationships of the Jurassic mammals 
to those of the Lance Creek beds and of the latter to the mammals of the 
Puerco and Torrejon. If it shall result that all, or nearly all, of the Lance 
Creek mammals belonged to the Marsupialia and the Monotremata, then 
Marsh’s opinion will be in great measure justified. If, on the other hand, 
it shall be shown hereafter that a large number of the Lance Creek mam- 
mals were placentals and the near-by ancestors of the Puerco and Torre- 
jon faunas the break between the former and the latter will not be a pro- 
found one; nevertheless more important than formerly supposed by Os- 
born. 
