WHITE] ANCESTRAL ORIGIN OF UNIONID 85 
Measured geologically, the life-time of species as such has been 
short, but genera, and the types which they embrace, have per- 
sisted through successive geological ages. The types of Unio 
which are represented on the accompanying plates have been thus 
preserved, while the species which successively bore them became 
extinct in the successive geological opochs. They so much resemble 
certain members of the living Mississippi river fauna as to warrant 
the assumption that the fossil faunas represent the living fauna 
ancestrally. 
Many specimens of fossil shells of the Unionide have been dis- 
covered in the Triassic strata of New Mexico and Wyoming. All 
of them are very imperfect and of comparatively small size, but 
they unmistakably belong to the genus Unio. One of these Triassic 
specimens is represented by figure 1, plate xxvi. The specimens re- 
ferred to are the earliest of the certainly known examples of the 
Unionidz in North America, although certain shells found in De- 
vonian and Carboniferous rocks have been supposed to belong to 
that family. These Triassic shells are all of simple form, and none 
of them exhibits distinctive prototypal relationship to the living 
Mississippi River fauna. Their structure and shell texture, how- 
ever, clearly show that the genus Unio was fully established at 
that early period; and their wide distribution indicates that a large 
Unione fauna was then established. 
In all, seven species of the Unionide have been discovered in the 
fresh water Jurassic strata of Colorado, Wyoming and South 
Dakota. All of them belong to the genus Umio, and five of the seven 
species are represented on plates xxvr and xxvii. They are all of 
simple, plain types, none of them exhibiting any special relationship 
to the Unione fauna of the Mississippi, unless it be U. stewardi. It 
is, however, not improbable that all these species, as well as those 
found in Triassic strata, are ancestrally related to the simpler forms 
of the Mississippi fauna. 
While there evidently was a large representation of the Unionidze 
in the Triassic and Jurassic periods, it was in the closing period 
of Mesozoic time, the Cretaceous, that the family received an extra- 
ordinary development. This fact is shown by the discovery at 
numerous places within a large geographical area, and in several 
successive formations, of a large number and great variety of fossil 
species of Unio, and of the addition among them of a few species 
of Anodonta and Margaritana. The increased diversity of the 
Unionide in this period is also shown in the exhibition by many 
of the species of Unio of those peculiarities which I have designated 
