BY R. J. TILLYARD. 597 



an entirely irregular and variable system of very weakly chitin- 

 ised cross-veins, such as can just be made out with difficulty in 

 the oldest fossils, and is not to be taken as indicating that there 

 were any cross-veins fixed in constant positions. Indeed, I would 

 go so far as to express the opinion that Permian forms may 

 yet be discovered in which no cross-veins exist at all, beyond 

 a few in specially suitable positions, e.g. as supports beneath 

 the dichotomous forkings of Rs and M 1 _ i% 



(2) Though the cubito-median Y-vein disappeared very early 

 from the wings of this Order, by fusion of Cttj with M 1—i near 

 its origin, yet this formation is indicated in the pupal wing of 

 Chorista (23), and is present, fully formed, in the Triassie fossil 

 Stereochorista. It is therefore included in the Archetype. I 

 am also of opinion that Permian forms will be discovered show- 

 ing this Y-vein as complete as it is in Stereochorista, and thus 

 serving to link the Paramecoptera with the true Mecoptera. 



(3) The cubitus is two-branched throughout the Order, the 

 fork being close to the base of the wing. Ciij is a strong, 

 straight, convex vein without any distal forking. Cu 2 is a 

 weak, concave vein, lying in or near the anal furrow. 



(4) A single line of descent is clearly evident, with very little 

 change during many millions of years, from the Permian genus 

 Permochorista, through the Triassie Mesochorista, to the still exist- 

 ing Taeniochorista, with Chorista itself as a specialised offshoot. 

 Apart from the changes already indicated in (1) above, the 

 only alteration of importance is the reduction in the number of 

 dichotomic branches of ^i__4 in the recent genera, which have 

 only five such branches in the f orewing, and four in the hind . 

 In the fossil genera, this vein has six branches, all of which are 

 clearly primitive dichotomies. Thus the Archetype of the Order 

 must also have at least six true dichotomic branches of M 1 _ 4 , as 

 in Text-fig. 60. The manner by which reduction has been 

 brought about is quite evident. The first fork to be eliminated 

 was that of M 4 , the result being the condition now extant in 

 the forewings of Taeniochorista and Chorista. Further reduc- 

 tion, in the hindwings only, led to the loss of the fork of M 2 . 

 This condition of a four-branched Mi_ 4 persists in all recent 

 winged forms within the Order. 



(5) As contrasted with the Australian Choristidae, the forms 

 known from the Northern Hemisphere show a progressive reduc- 

 tion not only in the number of dichotomic branches of M 1—4 



