80 SMITHSONIAN MISCELLANEOUS COLLECTIONS [voL. 48 
fossil faunas are ancestrally related to the living fauna of the Mis- 
sissippi River; and by inference that other living faunas had a like 
ancestral origin. 
Of late years students of the living North American Unionidze 
have recognized among the abundant species a considerable number 
of genera; properly basing their determinations largely upon the 
structure of the animal itself as well as upon that of the shell, and 
also to some extent upon group-differences that were formerly much 
overlooked. The earlier North American naturalists, however, clas- 
sifying those mollusks by means of the shells alone, usually recog- 
nized only three genera, namely, Unio, Anodonta and Margaritana 
or Alasmidonta, many of them having regarded the latter name as 
only a synonym of Margaritana. In studies of the fossil Unionide 
one is necessarily confined to the shells alone; and the fossil material 
which is available is usually insufficient for the recognition of such 
groups of species as are recognizable among living faunas. Because 
of these facts, and partly from a long established habit, I have 
retained that older classification in my studies of the fossil species. 
As the character of this article does not really require it, I do not 
now make any special reference to the improved classification. 
Although the Mississippi fauna contains about four hundred spe- 
cies only about a dozen of them are referred to the genus Anodonta. 
Their preferred habitat is in still waters apart from the two other 
genera, and their shells are all of plain, simple type. The species 
that are referable to the genus Margaritana, including Alasmuidonta, 
in the same fauna are less in number than are those of Anodonta. 
They live in immediate association with Unio, and their shells have 
considerable diversity of form and surface features. It is therefore 
almost only among the teeming species of Unio that occurs the 
great variety of form and surface features by which the shells of 
these mollusks have given expression to what naturalists have long 
recognized as North American types of the Unionidz. I shall show 
that this term is properly so applied, not only because these Missis- 
sippi River types are different from those which are found among 
the living members of the family in other parts of the world, but 
because they evidently have been derived from ancient North Amer- 
ican ancestry. The illustrations upon the accompanying plates ex- 
press this prototypal character of the fossil species, so far as is 
practicable by such means, with the aid of the material that has 
hitherto been discovered.t It is only claimed that the expression 
*The specimens from which these figures were drawn are all the property 
of the U. S. National Museum. 
