feY H. ,f. TILLYARD. 241 



from the true cross-veins. We have, then, a test in this fossil : 

 does the posterior arculus carry macrotrichia or not 1 The basal 

 two- fifths of this vein are unfortunately not very well preserved, 

 but the remainder is in excellent preservation, and the bases of 

 at least two macrotrichia can be seen upon it. One of these is 

 very beautifully preserved, and is- indicated by the large arrow 

 placed just below it in Plate xiii., fig. 3. The other can just be 

 made out, a little higher up, in the same Plate, but is not very 

 distinct. Similar bases of macrotrichia can be seen upon the 

 other main veins of the wing- a particularly good one can be 

 picked up on Cuj by following the main stem of the same arrow 

 in Plate xiii., fig. 3, backwards. These bases are more abundant 

 and better preserved in the apical part of the wing, where the 

 veins are, on the whole, more clearly defined. 



It seems evident, therefore, that the posterior arculus must be 

 rightly considered as having been originally M 5 , and the exact 

 homologue of M 4 in the Zygopterous Odonata. If so, then the 

 vein that we now call Cuj may well be actually M 5 + Cuj, and its 

 distal forkings may be due to incomplete fusion of the two veins 

 M 5 and Cu distally, Cu ]a being really the distal portion of M 5 , 

 and Cu lb the distal portion of Cu^ It should be noted that the 

 term posterior arculus is a misnomer, if this be really the 

 case; for it is the anterior part of the arculus in Odonata that is 

 formed from M, the posterior part being a cross-vein. Also the 

 point of origin of M 5 should be rightly designated the primary 

 median fork (mf), while the point at which M x _4 divides into 

 M 1+2 and M 3 _i_ 4 , hitherto designated mf (sometimes called the 

 thyridium), must be regarded as the secondary median for k(iuf ). 



I would suggest that the term posterior arculus should be no 

 longer used for the vein in question, and that it be simply desig- 

 nated the arculus, with the notation arc or M 6 . 



An examination of the arculus in the Caddis-fly Rhyacophila 

 dorsalis Curtis, reveals the fact that, in this insect also, this vein 

 carries macrotrichia. This is in agreement with the evidence 

 from the fossil, so that there can really be little doubt that we 

 are dealing with a true branch of the media. 



But, whatever view we may take as regards the true nature of 



