246 
Platygenesis has taken place in one group and stenogenesis in 
another. The aedeagus has undergone considerable evolution 
within the family, but, so far as present knowledge enables us 
to judge, it is of the Cixiinae type. The generalized Derbidae 
approach so closely to the Cixiidae that some genera have been 
shifted from one to another, but they possess distinct male geni- 
talia. The Achilixiidae belong to the Cixiinae group, but they 
are difficult to place as they have some synthetic characters. 
The other nine families arose from the Meenoplinae stock or 
from genera having their type of genitalia. Our knowledge is 
too slight to allow us to speculate on their relationship with any 
hope of being correct. The Dictyopharidae and Fulgoridae are 
closely allied. The Eurybrachidae show some affinities to the 
Fulgoridae, and so do the Achilidae. ‘The other five families 
may have arisen from the generalized Dictyopharidae or from 
a more direct Meenoplinae stem. The Issidae and Acanaloniidae 
are closely allied, but the possibility of the Issidae being a com- 
posite group must not be overlooked. ‘The Lophopidae and the 
Ricaniidae have affinities, and the Flatidae come close to them. 
With the exception of the Issidae, one of whose characteristics 
is a reduction and thickening of the tegmina, the last five 
families show considerable platygenesis, the last three often hav- 
ing a wide costal area containing cross-veins; the Tropiduchidae 
in part share this characteristic, and other families show it to 
some extent. Stenogenesis also appears in several families quite 
independently, so that neither of these characters can be used 
for the grouping of the families. 
It is to paleontology that we must look for information to fill 
in our time elements so as to round out our speculations in 
phylogeny. So far the evidence fits in with the above conclu- 
sions. The Tropiduchidae, Cixiidae, and Tettigometridae (if 
the latter are allied to /psvichia) are found in Mesozoic times 
or earlier, along with Cercopidae, Cicadidae, and Cicadellidae ; 
whereas Fulgoridae, Flatidae, Ricaniidae and, perhaps, the Der- 
bidae have only been found in much more recent formations. 
While paleontology gives us some positive data as to the pres- 
ence of certain forms at certain periods, yet the geological 
record is not nearly complete enough to allow us to accept nega- 
tive evidence as indicating that other forms did not exist at 
