56.4.1U (118:78.7) 
Article IV.— NEW SPECIES OF UNIO FROM THE TERTIARY 
ROCKS OF WYOMING. 
By T. D. A. CocKxereE .u. 
Among the rather numerous specimens of Unio (sens. latiss.) collected 
by the American Museum expeditions in Wyoming, I find material rep- 
resenting four apparently new species. Although the taxonomy of the 
Unionidee, even with living species, is so difficult as to give rise to endless 
controversy, | have some confidence in the validity of the species now 
described. At the National Museum, thanks to the kindness of Dr. Dall, 
I have been able to study all the previously described Eocene (exclusive of 
Paleocene) Rocky Mountain species, with the exception of U. rectoides 
White. The Fort Union beds (Paleocene) contain a very rich and varied 
Unionid fauna, consisting of 23 known species, described by Whitfield, 
White and Meek. It is an astonishing fact that this fauna, in its most 
characteristic elements, disappears somewhere about the middle of the Fort 
Union (as understood by Knowlton '), at about the same time as the dino- 
saurs. The dinosaurs have disappeared from the earth, but the Unionid 
fauna of the early Fort Union, in all its major features, survives in great 
abundance in the Mississippi Valley. In the Rocky Mountain region it has 
entirely gone, except that it has spread west to some extent in the rivers 
crossing the plains; this disappearance took place long ago, leaving in 
the Eocene only a few generic or subgeneric units, which eventually died 
out. Knowlton (Proc. Wash. Acad. Sci., XI, p. 232), discussing the possi- 
ble reasons for the disappearance of the dinosaurs, concludes: “no more 
plausible theory occurs to the writer than that they were suddenly removed 
by epidemic disease, so many examples of which among recent animals have 
been given by Professor Osborn.”’ This, obviously, will not explain the case 
of the Unionide, and we naturally think rather of movements of the earth, 
which drained the extensive waterways and produced conditions equally 
unfavorable to the Unionids and the dinosaurs. Knowlton, however, says 
“the waters were not drained; for sedimentation was continuous.” It must 
be recalled that a very slight change of level would suffice to produce great 
changes in the water systems, and it certainly appears that the vast mass of 
1 [t is not intended to express any particular opinion as to the lower limit of the Tertiary 
in using Knowlton’s nomenclature. There appears to be no sharp line of demarcation 
between the Cretaceous and Tertiary in the Rocky Mountains, and since the limit thus 
becomes a matter for arbitrary decision, the question has no bearing on the biological 
argument. 
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