BY R. J. TILLY A RD. 



699 



archetypic definition. It has a pectinate Rs, and both M and 

 Cu are three-branched . 



It will be realised, from Text-tig. 108. that the Archetype of 

 the Megaloptera possesses a very primitive type of venation; 

 and that it is older, in some respects, than the Permian Bel- 

 montia. This speaks for a very ancient origin for the Order. 

 The question is more fully discussed 'in Section xvii. 



Section xvi. The Venation of the Planipennia. 

 (Text-figs. 36a, 39-40. 45, 52, 109-110.) 



This Order is the dominant one ' amongst the Neuropteroidea, 

 and exhibits a wealth of venation al specialisations not equalled 

 in any other known Order, except perhaps in the Odonata. To 

 deal with all these would take us far beyond the limits set down 

 in this Part. 1 shall confine myself here strictly to the con- 

 struction of the Archetype of the Order, while leaving a fuller 

 discussion of the internal phylogeny of the different families for 

 a later paper. In this connection, it is only necessary to refer 

 the reader to the excellent figures and descriptions given by 

 Comstock (15, chap, ix. ), which will be mentioned also when 

 certain types are being discussed. 



The fossil record of the Planipennia. unlike that of the Mega- 



Text-Fig. 109. 

 Forewing of an archaic representative of the family Prohemerobiidae, 

 with -R4+5 still dichotomously branched. The same type will stand 

 for the Archetype of the Order Planipennia. Cross-veins omitted, 

 except only those between Sc and R], and between Ei and Ks, which 

 are constant. Lettering as on p. 535. 



