1930 ] Wing Venation of the Odonata and Agnatha 255 
corresponds to the form of the fore plus hind wings in 
other groups, or to one wing of the homonomous group 
(Dictyoneuridse, Carboniferous and Permian Ephemerids). 
Therefore, I think that the primitiveness of Bsetoidea wings 
is only an apparent one and that in the past their fore wings 
were also probably of triangular shape. The differences 
in dimensions and in the shape of both pairs, in their turn 
also, were formed for a second time, replacing the original 
homonomous condition. The venation inherited from the 
ancestors was not effected by the change in the shape of 
the wings, and it remained very much like that of such 
may-flies of the Permian as Protereisma, Protechma, and 
also as Triplosoba. The homologizing of veins in these 
last forms with those in the Dictyoneuridse is therefore not 
difficult and is correctly interpreted by Handlirsch. RS in 
Triplosoba also turned off (from R) at the very base, but 
here the place of origin (how, is another question) is still 
nearer to the base. But more often Rs is derived (sec- 
ondarily) from the basal part of M.* The branching of 
RS is entirely similar to that of the Palaeozoic may-flies and 
to that of the majority of Dictyoneuridse, i. e., RS forms 
two groups : a basal branch, and another one almost always 
without a basal connection, the “inserted sector,” Rs 4 ; and 
the distal one, including RS 3 , RS 2 , and RSi, the sector 
RS 2 having already lost its true origin (“Schaltsector”) . 
In the hind wings, because of their reduction, the branches of 
RS are very seldom preserved, as in Palingenia; usually 
only RS 5 and RS 4 , and naturally RS 4 , are preserved, but 
RS 3 and RS 2 retreat towards the edge and are reduced. 
M corresponds to MP in the Triplosobidse and Dicty- 
neuridse, and forms the usual three branches. The branches 
of M in the hind wings are reduced also, and usually are 
simple (fig. 10). At the base of the fore wings, M ap- 
proaches R almost to contact, and weakening, disappears; 
in the hind wings M often fuses at the base with R. Into 
this narrow path RS is directed and usually diverges from 
M, but often ends here as an “inserted sector,” i. e., it 
weakens and disappears, being joined basally with R and 
M by means of the usual cross-veins. In the hind wings, 
* Misprinted M x in original. 
