; 
210 A. G. Wetherby—Distribution of Fresh-water Mollusks, — 
a significant fact that those North American rivers which con- 
tain the richest Unione fauna drain Mesozoic and Tertiary 
tropolis of these shells. And it is here that we find the two 
aunas above indicated most distinctly developed. The rivers 
draining the Mesozoic and Tertiary regions of the west have a 
very meagre fauna, both as to species and individuals; and I 
have already stated, that, with the exception of the few Ano- 
dontas of the northwest, the entire assemblage is composed of 
Ohio types. Until series of casts of the Ohio River shells are 
made, and these are carefully compared with the casts of species 
described from these western localities, we shall not have 
reached the best conclusion which a study of these fossils will 
afford us. If we consider the species of the Mesozoic and ‘Ter- 
tiary regions of the south and southwest, we shall find that 
when we have removed the Ohio types from the lists, very few 
valid species remain. How absolutely true this is, and how 
their remnants which have spread over the same area 
persistent species have either less tendency to variation, or the 
precise circumstances to call out such latent energies have not 
yet been brought into active account; while other forms, for 
opposite reasons, present us an infinity of varieties, always 
easily recognized, and of the derivative character of which no 
person, who has investigated this subject, can have any doubt. 
In this connection the isolated fauna of the Coosa, to which 
reference was made in the previous article, must not be neg- 
lected. This stream flows through a comparatively limited 
drainage. It contains two genera, Schizostoma and Tuiotoma, 
