1930 ] Wing Venation of the Odonata and Agnatha 267 
can say that the primitive net of their wing was used for 
the formation of secondary longitudinal sectors, which 
imitate and take the position of the original ones. In 
contrast to what was as a rule expressed in the evolution of 
the Palseodictyoptera, may-flies, and dragon-flies, Rs did 
not expand, but decreased. In these the distribution of 
branches of RS and also Mi and Cu is more pronounced than 
in the Odonata, and departs from the distribution which we 
find in the Dictyoneuridse and Protephemeroidea. In this 
respect the Odonata have preserved more of the primitive 
features. The one-sided and extreme specialization of the 
wing venation of the Protodonata, with the loss of some 
fundamental features of their ancestors, were probably one 
of the main causes which brought about the rapid extinction 
of the group. 
From the base of the Odonata there separated off, proba- 
bly some time before the adoption of the main features of 
the recent venation, one more group known to us through a 
single representative from the Liassic — Protomyrmelon 
brunonis Geinitz. The wing venation of this form was fig- 
ured by Handlirsch on PI. 42, Fig. 14, and is reproduced 
here in Fig. 8. Some superficial resemblances with the 
Zygoptera, and at the same time some singularities of vena- 
tion, lead the author to isolate the species in a separate sub- 
order, Archizygoptera. Sc is very much shortened and Rs 
and M very strongly distorted, so it is difficult to understand 
their distribution. Handlirsch represented Cu as two- 
branched ; below it came the anal. If Cu is actually composed 
of two branches, then this fact, together with the peculiar 
structure of RS and M, would be sufficient for us to place 
this form into a separate order by itself. But one should 
note that the base of the wing is not well preserved in the 
fossil. 
Now let us look briefly at the tracheation of the wings of 
the may-flies and dragon-flies. Such an inquiry is natural. If 
our interpretation of the venation of the dragon-flies and 
may-flies is correct, how can we explain, then, the way the 
trachea of RS runs in the anisoptera, where the trachea which 
arises from R is opposite the nodus, crosses RSi and RS 3 and 
enters into the vein RS 4 , extending, and often a great deal, 
from its base? How can we explain the almost regular 
