* 
L. Agassiz on Fishes of the Tennessee River. 363 
piodes, Ictiobus, Cycleptus, Catostomus, Rhinichthys, Chrosomus, 
Hypsolepis, Hybopsis which are foreign to the old world, and 
they share together the genera Alburnus, Chondrostoma, Leucis- 
cus, &c., still, with this difference, that the true Leucisci are far 
more numerous in the old world than in North America. In the 
family of Cyprinodonts we find exactly the reverse, there being 
in North America a much greater diversity and a larger number 
of representatives of this type than in the old world. ‘The case is 
still different with the family of the Etheostomoids ; which are 
altogether peculiar to North America, not a single species being 
known in the old world. The family of Ccelacanths is also en- 
tirely foreign to the old world, whilst the Sauroids are represented 
y one genus, Polypterus in the old world and by another, Lepi- 
dosteus in America. The Scienoids differ in another respect : 
whilst these fishes inhabit exclusively the sea in the old world, 
there are in North America besides many marine representatives, 
a number of fresh-water species constituting a distinct genus, Am- 
odon. Again the family of Siluroids, is represented by a great 
Variety of species in North America, and on a few in the 
old world. Similar facts might be mentioned of other families, 
but this may be sufficient to show how important it is to combine 
the study of the modifications of the structure of animals with 
that of their geographical distribution. 
For it is not the presence here or there of this or that species 
of any genus, or family or higher group which I would particu- 
larly consider in the study of the geographical distribution of 
organized beings, but the localization upon certain parts of the 
Surface of the globe of special modifications of definite types 
representing each a distinct idea, expressed in a variety of living 
orms and combined in various ways in time and space. 
There is another point of view of equal interest in this con- 
ection ; the mode of association of different families in different 
Parts of the world. It is a fact for instance that the Goniodonts 
are limited to South America, and that this family, which is en- 
tirely wanting in the old world, has no nearer relative than that ge- 
hus of Sturgeons peculiar to North America, the Scaphirhynehus. 
Again, whilst the families mentioned above as characteristic of 
the North American fresh-water fish fauna seem to be equally 
distributed over the surface of this vast continent, there is yeta 
Specia adaptation of some of their types to peculiar localities. 
The great similarity of their representatives throughout the 
Southern Atlantic States, the Gulf States and the Mississippi 
Valley, as high up as the Ohio, including even Lake Champlain, 
€s not extend to the New England States, which although en- 
circled by this uniform combination of fresh-water animals, have 
another zoological character, peculiar to itself, aud approximating 
Rore to that of the old world under the same cl 
