BY R. J. TILLYARD. 553 



Parameeoptera and Protomeeoptera, M,_ 4 bad more than four 

 dichotomic branches originally. It is equally clear that, in the 

 more specialised Orders Diptera, Trichoptera and Lepidoptera, 

 this number was reduced to four. As regards the Megaloptera 

 and Planipennia, the evidence is not conclusive one way or 

 another. For. although the great majority of forms in these 

 Orders have this vein reduced to four branches or less, yet the 

 fossil Kalligrammatidae show it with from five to nine branches, 

 while six branches can be counted in the hindwing of the recent 

 Megapsychops. But these types are both of them Planipennian, 

 and exceptionally abundantly veined. I have therefore assumed 

 that the Orders Megaloptera and Planipennia had archetypically 

 M,_ 4 with only four branches, though I think that more definite 

 evidence on this point may be forthcoming later on from the 

 fossil record . 



As regards the condition of M s it is clear that the complete 

 fusion of that vein with Cu, to form the cubito-median Y-vein, 

 as in Belmontia (Text-fig. 63), is a very primitive condition. 

 We are still in doubt, hoAvever, as to whether the state of this 

 vein shown in certain primitive Megaloptera and Planipennia 

 (Text-figs. 44. 45) is to be regarded as derivable from the con- 

 dition seen in Belmontia, or whether it is a reduction from a 

 more primitive formation, in which M. g has not succeeded in 

 becoming fused with Cu, Until we have further evidence on 

 that point, we cannot fix the geological age at which these Orders 

 arose. This question is fully discussed in Section iii. 



The Cubitus. 



A special Section ( Section iv. ) has been devoted to this vein, 

 as the condition of its branching is of the utmost importance 

 in this research. The conclusions reached are that Cu w 7 as 

 originally three-branched, On, having a distal forking into Cu, 

 and Cu'| b , while Cu 2 was a weak concave vein. It is possible 

 that the distal branching of Cu, may have been originally due to 

 incomplete fusion of M 5 distally with that vein; but this point 

 cannot be decided with certainty, in the absence of fossil evi- 

 dence. 



The primary cubital fork (cuf) is situated near the base of 

 the wing, and is the point at which Cu divides into Cu, and Cu, 

 The secondary cubital fork (cuf) is the point at which Cu, 

 divides into Cu, a i and Cu, b< This latter forking is present in 

 all the Orders of the Complex, except only in the Mecoptera, 



