166 DISTRIBUTION OF EXTINCT ANIMALS. {PART II. 
is allied to the Menopoma living in North America. Species 
of frog (Rana), and Paleophryus an extinct genus of toads, 
have been found in the Miocene deposits of Germany and 
Switzerland. 
Fresh water fish are almost unknown in the Tertiary 
deposits of Europe, although most of the families and some 
genera of living marine fish are represented from the Eocene 
downwards. 
ANTIQUITY OF THE GENERA OF INSECTS. 
Fossil insects are far too rarely found, to aid us in our determi- 
nation of difficult questions of geographical distribution ; but in 
discussing these questions it will be important to know, whether 
we are to look upon the existing generic forms of insects as of 
great or small antiquity, compared with the higher vertebrates ; 
and to decide this question the materials at our command are 
ample. 
The conditions requisite for the preservation of insects in a 
fossil state are no doubt very local and peculiar ; the result being, 
that it is only at long intervals in the geological record that we 
meet with remains of insects in a recognisable condition. None 
appear to have been found in the Pliocene formation ; but in the 
Upper Miocene of (Eninghen in Switzerland, associated with the 
wonderfully rich fossil flora, are found immense quantities of 
insects. Prof. Heer examined more than 5,000 specimens be- 
longing to over 800 species, and many have been found in other 
localities in Switzerland; so that more than 1,300 species of 
Miocene insects have now been determined. Most of the orders 
are represented, but the beetles (Coleoptera) are far the most 
abundant. Almost all belong to existing genera, and the majority 
of these genera now inhabit Europe, only three or four being 
exclusively Indian, African, or American. 
In the Lower Miocene of Croatia there is another rich de- 
posit of insects, somewhat more tropical in character, comprising 
large white-ants and dragon-flies differently marked from any 
