1913.] NATURAL SCIENCES OF PHILADELPHIA. 245 



None of the following Old World genera seem to offer any close 

 approximations to Phenacolestes and therefore have not been studied 

 further in this connection: Neurolestes Selys, Nesolestes Selys, 

 Podolestes Selys and Synlestes Selys. It may be notecl, however, 

 that in Neurolestes trinervis Selys, from Old Calabar, the three 

 antenodals are so arranged that the arculus is at the third antenodal 

 on all four wings. 



General Discussion of the Value of the Venational Charac- 

 ters COMPARED IN THE PRECEDING GeNERA FOR THE 



Determination of Relationships. 



Many of the ideas which have been expressed on the changes 

 which have taken place in the venation of Odonate wings liave been 

 based on the comparative morphology of living representatives of 

 the order or on considerations of mechanical advantage. It does 

 not follow that these foundations are trustworthy or that there has 

 always been mechanical improvement in the wings. Degeneration 

 is just as probable as progressive development. The actual course 

 of phylogeny cannot be deduced from these considerations or from 

 the data of morphology. It seems therefore desirable to attempt to 

 ascertain what paleontological evidence exists affording clues to the 

 descent of these insects, by tracing the modifications which the wings 

 exhibit from the Carboniferous period down to the present time. 



The characters of Phenacolestes and other genera which have been 

 compared in the preceding pages under the numbers 1 to 22 may for 

 the most part be arranged into two groups: 



A. Those characters which are concerned with the positions of 

 the points of separation of longitudinal veins and the positions and 

 number of certain cross-veins. 



B. Those characters which are concerned with the number of rows 

 of cells existing between the longitudinal veins and their branches. 



Group A includes numbers 1, 2, 3, 4, 9, 11, 13, 14, 15r/, 16, 17a, 

 18, 21, and 22. 



1. The Position of the Nodus. — The Protodonata of the Carbonifer- 

 ous and the Permian possessed no nodus ; their subcosta (whose apex 

 in the Odonata usually coincides with the nodus) reached often beyond 

 mid-length of the wing, its minimal extent being to four-tenths of 

 the wing-length in Meganeurula (Handlirsch'^'-). The earliest of 



52 Foss. Ins., p. 309, Taf. XXXI, figs. 37, 38. 



